FreeBSD/Linux Kernel Cross Reference
sys/xen/xenstore/xenstore.c

     /******************************************************************************
      * xenstore.c
      *
      * Low-level kernel interface to the XenStore.
      *
      * Copyright (C) 2005 Rusty Russell, IBM Corporation
      * Copyright (C) 2009,2010 Spectra Logic Corporation
      *
      * This file may be distributed separately from the Linux kernel, or
     * incorporated into other software packages, subject to the following license:
     *
     * Permission is hereby granted, free of charge, to any person obtaining a copy
     * of this source file (the "Software"), to deal in the Software without
     * restriction, including without limitation the rights to use, copy, modify,
     * merge, publish, distribute, sublicense, and/or sell copies of the Software,
     * and to permit persons to whom the Software is furnished to do so, subject to
     * the following conditions:
     *
     * The above copyright notice and this permission notice shall be included in
     * all copies or substantial portions of the Software.
     *
     * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
     * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
     * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
     * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
     * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
     * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
     * IN THE SOFTWARE.
     */
    
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include <sys/param.h>
    #include <sys/bus.h>
    #include <sys/kernel.h>
    #include <sys/lock.h>
    #include <sys/module.h>
    #include <sys/mutex.h>
    #include <sys/sx.h>
    #include <sys/syslog.h>
    #include <sys/malloc.h>
    #include <sys/systm.h>
    #include <sys/proc.h>
    #include <sys/kthread.h>
    #include <sys/sbuf.h>
    #include <sys/sysctl.h>
    #include <sys/uio.h>
    #include <sys/unistd.h>
    
    #include <machine/xen/xen-os.h>
    #include <machine/stdarg.h>
    
    #include <xen/evtchn.h>
    #include <xen/gnttab.h>
    #include <xen/hypervisor.h>
    #include <xen/xen_intr.h>
    
    #include <xen/interface/hvm/params.h>
    
    #include <xen/xenstore/xenstorevar.h>
    #include <xen/xenstore/xenstore_internal.h>
    
    #include <vm/vm.h>
    #include <vm/pmap.h>
    
    /**
     * \file xenstore.c
     * \brief XenStore interface
     *
     * The XenStore interface is a simple storage system that is a means of
     * communicating state and configuration data between the Xen Domain 0
     * and the various guest domains.  All configuration data other than
     * a small amount of essential information required during the early
     * boot process of launching a Xen aware guest, is managed using the
     * XenStore.
     *
     * The XenStore is ASCII string based, and has a structure and semantics
     * similar to a filesystem.  There are files and directories, the directories
     * able to contain files or other directories.  The depth of the hierachy
     * is only limited by the XenStore's maximum path length.
     *
     * The communication channel between the XenStore service and other
     * domains is via two, guest specific, ring buffers in a shared memory
     * area.  One ring buffer is used for communicating in each direction.
     * The grant table references for this shared memory are given to the
     * guest either via the xen_start_info structure for a fully para-
     * virtualized guest, or via HVM hypercalls for a hardware virtualized
     * guest.
     *
     * The XenStore communication relies on an event channel and thus
     * interrupts.  For this reason, the attachment of the XenStore
     * relies on an interrupt driven configuration hook to hold off
     * boot processing until communication with the XenStore service
     * can be established.
     *
     * Several Xen services depend on the XenStore, most notably the
     * XenBus used to discover and manage Xen devices.  These services
    * are implemented as NewBus child attachments to a bus exported
    * by this XenStore driver.
    */
   
   static struct xs_watch *find_watch(const char *token);
   
   MALLOC_DEFINE(M_XENSTORE, "xenstore", "XenStore data and results");
   
   /**
    * Pointer to shared memory communication structures allowing us
    * to communicate with the XenStore service.
    *
    * When operating in full PV mode, this pointer is set early in kernel
    * startup from within xen_machdep.c.  In HVM mode, we use hypercalls
    * to get the guest frame number for the shared page and then map it
    * into kva.  See xs_init() for details.
    */
   struct xenstore_domain_interface *xen_store;
   
   /*-------------------------- Private Data Structures ------------------------*/
   
   /**
    * Structure capturing messages received from the XenStore service.
    */
   struct xs_stored_msg {
           TAILQ_ENTRY(xs_stored_msg) list;
   
           struct xsd_sockmsg hdr;
   
           union {
                   /* Queued replies. */
                   struct {
                           char *body;
                   } reply;
   
                   /* Queued watch events. */
                   struct {
                           struct xs_watch *handle;
                           const char **vec;
                           u_int vec_size;
                   } watch;
           } u;
   };
   TAILQ_HEAD(xs_stored_msg_list, xs_stored_msg);
   
   /**
    * Container for all XenStore related state.
    */
   struct xs_softc {
           /** Newbus device for the XenStore. */
           device_t xs_dev;
   
           /**
            * Lock serializing access to ring producer/consumer
            * indexes.  Use of this lock guarantees that wakeups
            * of blocking readers/writers are not missed due to
            * races with the XenStore service.
            */
           struct mtx ring_lock;
   
           /*
            * Mutex used to insure exclusive access to the outgoing
            * communication ring.  We use a lock type that can be
            * held while sleeping so that xs_write() can block waiting
            * for space in the ring to free up, without allowing another
            * writer to come in and corrupt a partial message write.
            */
           struct sx request_mutex;
   
           /**
            * A list of replies to our requests.
            *
            * The reply list is filled by xs_rcv_thread().  It
            * is consumed by the context that issued the request
            * to which a reply is made.  The requester blocks in
            * xs_read_reply().
            *
            * /note Only one requesting context can be active at a time.
            *       This is guaranteed by the request_mutex and insures
            *       that the requester sees replies matching the order
            *       of its requests.
            */
           struct xs_stored_msg_list reply_list;
   
           /** Lock protecting the reply list. */
           struct mtx reply_lock;
   
           /**
            * List of registered watches.
            */
           struct xs_watch_list  registered_watches;
   
           /** Lock protecting the registered watches list. */
           struct mtx registered_watches_lock;
   
           /**
            * List of pending watch callback events.
            */
           struct xs_stored_msg_list watch_events;
   
           /** Lock protecting the watch calback list. */
           struct mtx watch_events_lock;
   
           /**
            * Sleepable lock used to prevent VM suspension while a
            * xenstore transaction is outstanding.
            *
            * Each active transaction holds a shared lock on the
            * suspend mutex.  Our suspend method blocks waiting
            * to acquire an exclusive lock.  This guarantees that
            * suspend processing will only proceed once all active
            * transactions have been retired.
            */
           struct sx suspend_mutex;
   
           /**
            * The processid of the xenwatch thread.
            */
           pid_t xenwatch_pid;
   
           /**
            * Sleepable mutex used to gate the execution of XenStore
            * watch event callbacks.
            *
            * xenwatch_thread holds an exclusive lock on this mutex
            * while delivering event callbacks, and xenstore_unregister_watch()
            * uses an exclusive lock of this mutex to guarantee that no
            * callbacks of the just unregistered watch are pending
            * before returning to its caller.
            */
           struct sx xenwatch_mutex;
   
   #ifdef XENHVM
           /**
            * The HVM guest pseudo-physical frame number.  This is Xen's mapping
            * of the true machine frame number into our "physical address space".
            */
           unsigned long gpfn;
   #endif
   
           /**
            * The event channel for communicating with the
            * XenStore service.
            */
           int evtchn;
   
           /** Interrupt number for our event channel. */
           u_int irq;
   
           /**
            * Interrupt driven config hook allowing us to defer
            * attaching children until interrupts (and thus communication
            * with the XenStore service) are available.
            */
           struct intr_config_hook xs_attachcb;
   };
   
   /*-------------------------------- Global Data ------------------------------*/
   static struct xs_softc xs;
   
   /*------------------------- Private Utility Functions -----------------------*/
   
   /**
    * Count and optionally record pointers to a number of NUL terminated
    * strings in a buffer.
    *
    * \param strings  A pointer to a contiguous buffer of NUL terminated strings.
    * \param dest     An array to store pointers to each string found in strings.
    * \param len      The length of the buffer pointed to by strings.
    *
    * \return  A count of the number of strings found.
    */
   static u_int
   extract_strings(const char *strings, const char **dest, u_int len)
   {
           u_int num;
           const char *p;
   
           for (p = strings, num = 0; p < strings + len; p += strlen(p) + 1) {
                   if (dest != NULL)
                           *dest++ = p;
                   num++;
           }
   
           return (num);
   }
   
   /**
    * Convert a contiguous buffer containing a series of NUL terminated
    * strings into an array of pointers to strings.
    *
    * The returned pointer references the array of string pointers which
    * is followed by the storage for the string data.  It is the client's
    * responsibility to free this storage.
    *
    * The storage addressed by strings is free'd prior to split returning.
    *
    * \param strings  A pointer to a contiguous buffer of NUL terminated strings.
    * \param len      The length of the buffer pointed to by strings.
    * \param num      The number of strings found and returned in the strings
    *                 array.
    *
    * \return  An array of pointers to the strings found in the input buffer.
    */
   static const char **
   split(char *strings, u_int len, u_int *num)
   {
           const char **ret;
   
           /* Protect against unterminated buffers. */
           strings[len - 1] = '\0';
   
           /* Count the strings. */
           *num = extract_strings(strings, /*dest*/NULL, len);
   
           /* Transfer to one big alloc for easy freeing by the caller. */
           ret = malloc(*num * sizeof(char *) + len, M_XENSTORE, M_WAITOK);
           memcpy(&ret[*num], strings, len);
           free(strings, M_XENSTORE);
   
           /* Extract pointers to newly allocated array. */
           strings = (char *)&ret[*num];
           (void)extract_strings(strings, /*dest*/ret, len);
   
           return (ret);
   }
   
   /*------------------------- Public Utility Functions -------------------------*/
   /*------- API comments for these methods can be found in xenstorevar.h -------*/
   struct sbuf *
   xs_join(const char *dir, const char *name)
   {
           struct sbuf *sb;
   
           sb = sbuf_new_auto();
           sbuf_cat(sb, dir);
           if (name[0] != '\0') {
                   sbuf_putc(sb, '/');
                   sbuf_cat(sb, name);
           }
           sbuf_finish(sb);
   
           return (sb);
   }
   
   /*-------------------- Low Level Communication Management --------------------*/
   /**
    * Interrupt handler for the XenStore event channel.
    *
    * XenStore reads and writes block on "xen_store" for buffer
    * space.  Wakeup any blocking operations when the XenStore
    * service has modified the queues.
    */
   static void
   xs_intr(void * arg __unused /*__attribute__((unused))*/)
   {
   
           /*
            * Hold ring lock across wakeup so that clients
            * cannot miss a wakeup.
            */
           mtx_lock(&xs.ring_lock);
           wakeup(xen_store);
           mtx_unlock(&xs.ring_lock);
   }
   
   /**
    * Verify that the indexes for a ring are valid.
    *
    * The difference between the producer and consumer cannot
    * exceed the size of the ring.
    *
    * \param cons  The consumer index for the ring to test.
    * \param prod  The producer index for the ring to test.
    *
    * \retval 1  If indexes are in range.
    * \retval 0  If the indexes are out of range.
    */
   static int
   xs_check_indexes(XENSTORE_RING_IDX cons, XENSTORE_RING_IDX prod)
   {
   
           return ((prod - cons) <= XENSTORE_RING_SIZE);
   }
   
   /**
    * Return a pointer to, and the length of, the contiguous
    * free region available for output in a ring buffer.
    *
    * \param cons  The consumer index for the ring.
    * \param prod  The producer index for the ring.
    * \param buf   The base address of the ring's storage.
    * \param len   The amount of contiguous storage available.
    *
    * \return  A pointer to the start location of the free region.
    */
   static void *
   xs_get_output_chunk(XENSTORE_RING_IDX cons, XENSTORE_RING_IDX prod,
       char *buf, uint32_t *len)
   {
   
           *len = XENSTORE_RING_SIZE - MASK_XENSTORE_IDX(prod);
           if ((XENSTORE_RING_SIZE - (prod - cons)) < *len)
                   *len = XENSTORE_RING_SIZE - (prod - cons);
           return (buf + MASK_XENSTORE_IDX(prod));
   }
   
   /**
    * Return a pointer to, and the length of, the contiguous
    * data available to read from a ring buffer.
    *
    * \param cons  The consumer index for the ring.
    * \param prod  The producer index for the ring.
    * \param buf   The base address of the ring's storage.
    * \param len   The amount of contiguous data available to read.
    *
    * \return  A pointer to the start location of the available data.
    */
   static const void *
   xs_get_input_chunk(XENSTORE_RING_IDX cons, XENSTORE_RING_IDX prod,
       const char *buf, uint32_t *len)
   {
   
           *len = XENSTORE_RING_SIZE - MASK_XENSTORE_IDX(cons);
           if ((prod - cons) < *len)
                   *len = prod - cons;
           return (buf + MASK_XENSTORE_IDX(cons));
   }
   
   /**
    * Transmit data to the XenStore service.
    *
    * \param tdata  A pointer to the contiguous data to send.
    * \param len    The amount of data to send.
    *
    * \return  On success 0, otherwise an errno value indicating the
    *          cause of failure.
    *
    * \invariant  Called from thread context.
    * \invariant  The buffer pointed to by tdata is at least len bytes
    *             in length.
    * \invariant  xs.request_mutex exclusively locked.
    */
   static int
   xs_write_store(const void *tdata, unsigned len)
   {
           XENSTORE_RING_IDX cons, prod;
           const char *data = (const char *)tdata;
           int error;
   
           sx_assert(&xs.request_mutex, SX_XLOCKED);
           while (len != 0) {
                   void *dst;
                   u_int avail;
   
                   /* Hold lock so we can't miss wakeups should we block. */
                   mtx_lock(&xs.ring_lock);
                   cons = xen_store->req_cons;
                   prod = xen_store->req_prod;
                   if ((prod - cons) == XENSTORE_RING_SIZE) {
                           /*
                            * Output ring is full. Wait for a ring event.
                            *
                            * Note that the events from both queues
                            * are combined, so being woken does not
                            * guarantee that data exist in the read
                            * ring.
                            *
                            * To simplify error recovery and the retry,
                            * we specify PDROP so our lock is *not* held
                            * when msleep returns.
                            */
                           error = msleep(xen_store, &xs.ring_lock, PCATCH|PDROP,
                                "xbwrite", /*timeout*/0);
                           if (error && error != EWOULDBLOCK)
                                   return (error);
   
                           /* Try again. */
                           continue;
                   }
                   mtx_unlock(&xs.ring_lock);
   
                   /* Verify queue sanity. */
                   if (!xs_check_indexes(cons, prod)) {
                           xen_store->req_cons = xen_store->req_prod = 0;
                           return (EIO);
                   }
   
                   dst = xs_get_output_chunk(cons, prod, xen_store->req, &avail);
                   if (avail > len)
                           avail = len;
   
                   memcpy(dst, data, avail);
                   data += avail;
                   len -= avail;
   
                   /*
                    * The store to the producer index, which indicates
                    * to the other side that new data has arrived, must
                    * be visible only after our copy of the data into the
                    * ring has completed.
                    */
                   wmb();
                   xen_store->req_prod += avail;
   
                   /*
                    * notify_remote_via_evtchn implies mb(). The other side
                    * will see the change to req_prod at the time of the
                    * interrupt.
                    */
                   notify_remote_via_evtchn(xs.evtchn);
           }
   
           return (0);
   }
   
   /**
    * Receive data from the XenStore service.
    *
    * \param tdata  A pointer to the contiguous buffer to receive the data.
    * \param len    The amount of data to receive.
    *
    * \return  On success 0, otherwise an errno value indicating the
    *          cause of failure.
    *
    * \invariant  Called from thread context.
    * \invariant  The buffer pointed to by tdata is at least len bytes
    *             in length.
    *
    * \note xs_read does not perform any internal locking to guarantee
    *       serial access to the incoming ring buffer.  However, there
    *       is only one context processing reads: xs_rcv_thread().
    */
   static int
   xs_read_store(void *tdata, unsigned len)
   {
           XENSTORE_RING_IDX cons, prod;
           char *data = (char *)tdata;
           int error;
   
           while (len != 0) {
                   u_int avail;
                   const char *src;
   
                   /* Hold lock so we can't miss wakeups should we block. */
                   mtx_lock(&xs.ring_lock);
                   cons = xen_store->rsp_cons;
                   prod = xen_store->rsp_prod;
                   if (cons == prod) {
                           /*
                            * Nothing to read. Wait for a ring event.
                            *
                            * Note that the events from both queues
                            * are combined, so being woken does not
                            * guarantee that data exist in the read
                            * ring.
                            *
                            * To simplify error recovery and the retry,
                            * we specify PDROP so our lock is *not* held
                            * when msleep returns.
                            */
                           error = msleep(xen_store, &xs.ring_lock, PCATCH|PDROP,
                               "xbread", /*timeout*/0);
                           if (error && error != EWOULDBLOCK)
                                   return (error);
                           continue;
                   }
                   mtx_unlock(&xs.ring_lock);
   
                   /* Verify queue sanity. */
                   if (!xs_check_indexes(cons, prod)) {
                           xen_store->rsp_cons = xen_store->rsp_prod = 0;
                           return (EIO);
                   }
   
                   src = xs_get_input_chunk(cons, prod, xen_store->rsp, &avail);
                   if (avail > len)
                           avail = len;
   
                   /*
                    * Insure the data we read is related to the indexes
                    * we read above.
                    */
                   rmb();
   
                   memcpy(data, src, avail);
                   data += avail;
                   len -= avail;
   
                   /*
                    * Insure that the producer of this ring does not see
                    * the ring space as free until after we have copied it
                    * out.
                    */
                   mb();
                   xen_store->rsp_cons += avail;
   
                   /*
                    * notify_remote_via_evtchn implies mb(). The producer
                    * will see the updated consumer index when the event
                    * is delivered.
                    */
                   notify_remote_via_evtchn(xs.evtchn);
           }
   
           return (0);
   }
   
   /*----------------------- Received Message Processing ------------------------*/
   /**
    * Block reading the next message from the XenStore service and
    * process the result.
    *
    * \param type  The returned type of the XenStore message received.
    *
    * \return  0 on success.  Otherwise an errno value indicating the
    *          type of failure encountered.
    */
   static int
   xs_process_msg(enum xsd_sockmsg_type *type)
   {
           struct xs_stored_msg *msg;
           char *body;
           int error;
   
           msg = malloc(sizeof(*msg), M_XENSTORE, M_WAITOK);
           error = xs_read_store(&msg->hdr, sizeof(msg->hdr));
           if (error) {
                   free(msg, M_XENSTORE);
                   return (error);
           }
   
           body = malloc(msg->hdr.len + 1, M_XENSTORE, M_WAITOK);
           error = xs_read_store(body, msg->hdr.len);
           if (error) {
                   free(body, M_XENSTORE);
                   free(msg, M_XENSTORE);
                   return (error);
           }
           body[msg->hdr.len] = '\0';
   
           *type = msg->hdr.type;
           if (msg->hdr.type == XS_WATCH_EVENT) {
                   msg->u.watch.vec = split(body, msg->hdr.len,
                       &msg->u.watch.vec_size);
   
                   mtx_lock(&xs.registered_watches_lock);
                   msg->u.watch.handle = find_watch(
                       msg->u.watch.vec[XS_WATCH_TOKEN]);
                   if (msg->u.watch.handle != NULL) {
                           mtx_lock(&xs.watch_events_lock);
                           TAILQ_INSERT_TAIL(&xs.watch_events, msg, list);
                           wakeup(&xs.watch_events);
                           mtx_unlock(&xs.watch_events_lock);
                   } else {
                           free(msg->u.watch.vec, M_XENSTORE);
                           free(msg, M_XENSTORE);
                   }
                   mtx_unlock(&xs.registered_watches_lock);
           } else {
                   msg->u.reply.body = body;
                   mtx_lock(&xs.reply_lock);
                   TAILQ_INSERT_TAIL(&xs.reply_list, msg, list);
                   wakeup(&xs.reply_list);
                   mtx_unlock(&xs.reply_lock);
           }
   
           return (0);
   }
   
   /**
    * Thread body of the XenStore receive thread.
    *
    * This thread blocks waiting for data from the XenStore service
    * and processes and received messages.
    */
   static void
   xs_rcv_thread(void *arg __unused)
   {
           int error;
           enum xsd_sockmsg_type type;
   
           for (;;) {
                   error = xs_process_msg(&type);
                   if (error)
                           printf("XENSTORE error %d while reading message\n",
                               error);
           }
   }
   
   /*---------------- XenStore Message Request/Reply Processing -----------------*/
   /**
    * Filter invoked before transmitting any message to the XenStore service.
    *
    * The role of the filter may expand, but currently serves to manage
    * the interactions of messages with transaction state.
    *
    * \param request_msg_type  The message type for the request.
    */
   static inline void
   xs_request_filter(uint32_t request_msg_type)
   {
           if (request_msg_type == XS_TRANSACTION_START)
                   sx_slock(&xs.suspend_mutex);
   }
   
   /**
    * Filter invoked after transmitting any message to the XenStore service.
    *
    * The role of the filter may expand, but currently serves to manage
    * the interactions of messages with transaction state.
    *
    * \param request_msg_type     The message type for the original request.
    * \param reply_msg_type       The message type for any received reply.
    * \param request_reply_error  The error status from the attempt to send
    *                             the request or retrieve the reply.
    */
   static inline void
   xs_reply_filter(uint32_t request_msg_type,
       uint32_t reply_msg_type, int request_reply_error)
   {
           /*
            * The count of transactions drops if we attempted
            * to end a transaction (even if that attempt fails
            * in error), we receive a transaction end acknowledgement,
            * or if our attempt to begin a transaction fails.
            */
           if (request_msg_type == XS_TRANSACTION_END
            || (request_reply_error == 0 && reply_msg_type == XS_TRANSACTION_END)
            || (request_msg_type == XS_TRANSACTION_START
             && (request_reply_error != 0 || reply_msg_type == XS_ERROR)))
                   sx_sunlock(&xs.suspend_mutex);
   
   }
   
   #define xsd_error_count (sizeof(xsd_errors) / sizeof(xsd_errors[0]))
   
   /**
    * Convert a XenStore error string into an errno number.
    *
    * \param errorstring  The error string to convert.
    *
    * \return  The errno best matching the input string.
    *
    * \note Unknown error strings are converted to EINVAL.
    */
   static int
   xs_get_error(const char *errorstring)
   {
           u_int i;
   
           for (i = 0; i < xsd_error_count; i++) {
                   if (!strcmp(errorstring, xsd_errors[i].errstring))
                           return (xsd_errors[i].errnum);
           }
           log(LOG_WARNING, "XENSTORE xen store gave: unknown error %s",
               errorstring);
           return (EINVAL);
   }
   
   /**
    * Block waiting for a reply to a message request.
    *
    * \param type    The returned type of the reply.
    * \param len     The returned body length of the reply.
    * \param result  The returned body of the reply.
    *
    * \return  0 on success.  Otherwise an errno indicating the
    *          cause of failure.
    */
   static int
   xs_read_reply(enum xsd_sockmsg_type *type, u_int *len, void **result)
   {
           struct xs_stored_msg *msg;
           char *body;
           int error;
   
           mtx_lock(&xs.reply_lock);
           while (TAILQ_EMPTY(&xs.reply_list)) {
                   error = mtx_sleep(&xs.reply_list, &xs.reply_lock,
                       PCATCH, "xswait", hz/10);
                   if (error && error != EWOULDBLOCK) {
                           mtx_unlock(&xs.reply_lock);
                           return (error);
                   }
           }
           msg = TAILQ_FIRST(&xs.reply_list);
           TAILQ_REMOVE(&xs.reply_list, msg, list);
           mtx_unlock(&xs.reply_lock);
   
           *type = msg->hdr.type;
           if (len)
                   *len = msg->hdr.len;
           body = msg->u.reply.body;
   
           free(msg, M_XENSTORE);
           *result = body;
           return (0);
   }
   
   /**
    * Pass-thru interface for XenStore access by userland processes
    * via the XenStore device.
    *
    * Reply type and length data are returned by overwriting these
    * fields in the passed in request message.
    *
    * \param msg     A properly formatted message to transmit to
    *                the XenStore service.
    * \param result  The returned body of the reply.
    *
    * \return  0 on success.  Otherwise an errno indicating the cause
    *          of failure.
    *
    * \note The returned result is provided in malloced storage and thus
    *       must be free'd by the caller with 'free(result, M_XENSTORE);
    */
   int
   xs_dev_request_and_reply(struct xsd_sockmsg *msg, void **result)
   {
           uint32_t request_type;
           int error;
   
           request_type = msg->type;
           xs_request_filter(request_type);
   
           sx_xlock(&xs.request_mutex);
           if ((error = xs_write_store(msg, sizeof(*msg) + msg->len)) == 0)
                   error = xs_read_reply(&msg->type, &msg->len, result);
           sx_xunlock(&xs.request_mutex);
   
           xs_reply_filter(request_type, msg->type, error);
   
           return (error);
   }
   
   /**
    * Send a message with an optionally muti-part body to the XenStore service.
    *
    * \param t              The transaction to use for this request.
    * \param request_type   The type of message to send.
    * \param iovec          Pointers to the body sections of the request.
    * \param num_vecs       The number of body sections in the request.
    * \param len            The returned length of the reply.
    * \param result         The returned body of the reply.
    *
    * \return  0 on success.  Otherwise an errno indicating
    *          the cause of failure.
    *
    * \note The returned result is provided in malloced storage and thus
    *       must be free'd by the caller with 'free(*result, M_XENSTORE);
    */
   static int
   xs_talkv(struct xs_transaction t, enum xsd_sockmsg_type request_type,
       const struct iovec *iovec, u_int num_vecs, u_int *len, void **result)
   {
           struct xsd_sockmsg msg;
           void *ret = NULL;
           u_int i;
           int error;
   
           msg.tx_id = t.id;
           msg.req_id = 0;
           msg.type = request_type;
           msg.len = 0;
           for (i = 0; i < num_vecs; i++)
                   msg.len += iovec[i].iov_len;
   
           xs_request_filter(request_type);
   
           sx_xlock(&xs.request_mutex);
           error = xs_write_store(&msg, sizeof(msg));
           if (error) {
                   printf("xs_talkv failed %d\n", error);
                   goto error_lock_held;
           }
   
           for (i = 0; i < num_vecs; i++) {
                   error = xs_write_store(iovec[i].iov_base, iovec[i].iov_len);
                   if (error) {
                           printf("xs_talkv failed %d\n", error);
                           goto error_lock_held;
                   }
           }
   
           error = xs_read_reply(&msg.type, len, &ret);
   
   error_lock_held:
           sx_xunlock(&xs.request_mutex);
           xs_reply_filter(request_type, msg.type, error);
           if (error)
                   return (error);
   
           if (msg.type == XS_ERROR) {
                   error = xs_get_error(ret);
                   free(ret, M_XENSTORE);
                   return (error);
           }
   
           /* Reply is either error or an echo of our request message type. */
           KASSERT(msg.type == request_type, ("bad xenstore message type"));
   
           if (result)
                   *result = ret;
           else
                   free(ret, M_XENSTORE);
   
           return (0);
   }
   
   /**
    * Wrapper for xs_talkv allowing easy transmission of a message with
    * a single, contiguous, message body.
    *
    * \param t              The transaction to use for this request.
    * \param request_type   The type of message to send.
    * \param body           The body of the request.
    * \param len            The returned length of the reply.
    * \param result         The returned body of the reply.
    *
    * \return  0 on success.  Otherwise an errno indicating
    *          the cause of failure.
    *
    * \note The returned result is provided in malloced storage and thus
    *       must be free'd by the caller with 'free(*result, M_XENSTORE);
    */
   static int
   xs_single(struct xs_transaction t, enum xsd_sockmsg_type request_type,
       const char *body, u_int *len, void **result)
   {
           struct iovec iovec;
   
           iovec.iov_base = (void *)(uintptr_t)body;
           iovec.iov_len = strlen(body) + 1;
   
           return (xs_talkv(t, request_type, &iovec, 1, len, result));
   }
   
   /*------------------------- XenStore Watch Support ---------------------------*/
   /**
    * Transmit a watch request to the XenStore service.
    *
    * \param path    The path in the XenStore to watch.
    * \param tocken  A unique identifier for this watch.
    *
    * \return  0 on success.  Otherwise an errno indicating the
    *          cause of failure.
    */
   static int
   xs_watch(const char *path, const char *token)
   {
           struct iovec iov[2];
   
           iov[0].iov_base = (void *)(uintptr_t) path;
           iov[0].iov_len = strlen(path) + 1;
           iov[1].iov_base = (void *)(uintptr_t) token;
           iov[1].iov_len = strlen(token) + 1;
   
           return (xs_talkv(XST_NIL, XS_WATCH, iov, 2, NULL, NULL));
   }
   
   /**
    * Transmit an uwatch request to the XenStore service.
    *
    * \param path    The path in the XenStore to watch.
    * \param tocken  A unique identifier for this watch.
    *
    * \return  0 on success.  Otherwise an errno indicating the
    *          cause of failure.
    */
   static int
   xs_unwatch(const char *path, const char *token)
   {
           struct iovec iov[2];
   
           iov[0].iov_base = (void *)(uintptr_t) path;
           iov[0].iov_len = strlen(path) + 1;
           iov[1].iov_base = (void *)(uintptr_t) token;
           iov[1].iov_len = strlen(token) + 1;
   
           return (xs_talkv(XST_NIL, XS_UNWATCH, iov, 2, NULL, NULL));
   }
   
   /**
    * Convert from watch token (unique identifier) to the associated
    * internal tracking structure for this watch.
    *
    * \param tocken  The unique identifier for the watch to find.
    *
    * \return  A pointer to the found watch structure or NULL.
    */
   static struct xs_watch *
   find_watch(const char *token)
   {
           struct xs_watch *i, *cmp;
   
           cmp = (void *)strtoul(token, NULL, 16);
   
           LIST_FOREACH(i, &xs.registered_watches, list)
                   if (i == cmp)
                          return (i);
  
          return (NULL);
  }
  
  /**
   * Thread body of the XenStore watch event dispatch thread.
   */
  static void
  xenwatch_thread(void *unused)
  {
          struct xs_stored_msg *msg;
  
          for (;;) {
  
                  mtx_lock(&xs.watch_events_lock);
                  while (TAILQ_EMPTY(&xs.watch_events))
                          mtx_sleep(&xs.watch_events,
                              &xs.watch_events_lock,
                              PWAIT | PCATCH, "waitev", hz/10);
  
                  mtx_unlock(&xs.watch_events_lock);
                  sx_xlock(&xs.xenwatch_mutex);
  
                  mtx_lock(&xs.watch_events_lock);
                  msg = TAILQ_FIRST(&xs.watch_events);
                  if (msg)
                          TAILQ_REMOVE(&xs.watch_events, msg, list);
                  mtx_unlock(&xs.watch_events_lock);
  
                  if (msg != NULL) {
                          /*
                           * XXX There are messages coming in with a NULL
                           * XXX callback.  This deserves further investigation;
                           * XXX the workaround here simply prevents the kernel
                           * XXX from panic'ing on startup.
                           */
                          if (msg->u.watch.handle->callback != NULL)
                                  msg->u.watch.handle->callback(
                                          msg->u.watch.handle,
                                          (const char **)msg->u.watch.vec,
                                          msg->u.watch.vec_size);
                          free(msg->u.watch.vec, M_XENSTORE);
                          free(msg, M_XENSTORE);
                  }
  
                  sx_xunlock(&xs.xenwatch_mutex);
          }
  }
  
  /*----------- XenStore Configuration, Initialization, and Control ------------*/
  /**
   * Setup communication channels with the XenStore service.
   *
   * \return  On success, 0. Otherwise an errno value indicating the
   *          type of failure.
   */
  static int
  xs_init_comms(void)
  {
          int error;
  
          if (xen_store->rsp_prod != xen_store->rsp_cons) {
                  log(LOG_WARNING, "XENSTORE response ring is not quiescent "
                      "(%08x:%08x): fixing up\n",
                      xen_store->rsp_cons, xen_store->rsp_prod);
                  xen_store->rsp_cons = xen_store->rsp_prod;
          }
  
          if (xs.irq)
                  unbind_from_irqhandler(xs.irq);
  
          error = bind_caller_port_to_irqhandler(xs.evtchn, "xenstore",
              xs_intr, NULL, INTR_TYPE_NET, &xs.irq);
          if (error) {
                  log(LOG_WARNING, "XENSTORE request irq failed %i\n", error);
                  return (error);
          }
  
          return (0);
  }
  
  /*------------------ Private Device Attachment Functions  --------------------*/
  static void
  xs_identify(driver_t *driver, device_t parent)
  {
  
          BUS_ADD_CHILD(parent, 0, "xenstore", 0);
  }
  
  /**
   * Probe for the existance of the XenStore.
   *
   * \param dev
   */
  static int 
  xs_probe(device_t dev)
  {
          /*
           * We are either operating within a PV kernel or being probed
           * as the child of the successfully attached xenpci device.
           * Thus we are in a Xen environment and there will be a XenStore.
           * Unconditionally return success.
           */
          device_set_desc(dev, "XenStore");
          return (0);
  }
  
  static void
  xs_attach_deferred(void *arg)
  {
          xs_dev_init();
  
          bus_generic_probe(xs.xs_dev);
          bus_generic_attach(xs.xs_dev);
  
          config_intrhook_disestablish(&xs.xs_attachcb);
  }
  
  /**
   * Attach to the XenStore.
   *
   * This routine also prepares for the probe/attach of drivers that rely
   * on the XenStore.  
   */
  static int
  xs_attach(device_t dev)
  {
          int error;
  
          /* Allow us to get device_t from softc and vice-versa. */
          xs.xs_dev = dev;
          device_set_softc(dev, &xs);
  
          /*
           * This seems to be a layering violation.  The XenStore is just
           * one of many clients of the Grant Table facility.  It happens
           * to be the first and a gating consumer to all other devices,
           * so this does work.  A better place would be in the PV support
           * code for fully PV kernels and the xenpci driver for HVM kernels.
           */
          error = gnttab_init();
          if (error != 0) {
                  log(LOG_WARNING,
                      "XENSTORE: Error initializing grant tables: %d\n", error);
                  return (ENXIO);
          }
  
          /* Initialize the interface to xenstore. */
          struct proc *p;
  
  #ifdef XENHVM
          xs.evtchn = hvm_get_parameter(HVM_PARAM_STORE_EVTCHN);
          xs.gpfn = hvm_get_parameter(HVM_PARAM_STORE_PFN);
          xen_store = pmap_mapdev(xs.gpfn * PAGE_SIZE, PAGE_SIZE);
  #else
          xs.evtchn = xen_start_info->store_evtchn;
  #endif
  
          TAILQ_INIT(&xs.reply_list);
          TAILQ_INIT(&xs.watch_events);
  
          mtx_init(&xs.ring_lock, "ring lock", NULL, MTX_DEF);
          mtx_init(&xs.reply_lock, "reply lock", NULL, MTX_DEF);
          sx_init(&xs.xenwatch_mutex, "xenwatch");
          sx_init(&xs.request_mutex, "xenstore request");
          sx_init(&xs.suspend_mutex, "xenstore suspend");
          mtx_init(&xs.registered_watches_lock, "watches", NULL, MTX_DEF);
          mtx_init(&xs.watch_events_lock, "watch events", NULL, MTX_DEF);
          xs.irq = 0;
  
          /* Initialize the shared memory rings to talk to xenstored */
          error = xs_init_comms();
          if (error)
                  return (error);
  
          error = kproc_create(xenwatch_thread, NULL, &p, RFHIGHPID,
              0, "xenwatch");
          if (error)
                  return (error);
          xs.xenwatch_pid = p->p_pid;
  
          error = kproc_create(xs_rcv_thread, NULL, NULL,
              RFHIGHPID, 0, "xenstore_rcv");
  
          xs.xs_attachcb.ich_func = xs_attach_deferred;
          xs.xs_attachcb.ich_arg = NULL;
          config_intrhook_establish(&xs.xs_attachcb);
  
          return (error);
  }
  
  /**
   * Prepare for suspension of this VM by halting XenStore access after
   * all transactions and individual requests have completed.
   */
  static int
  xs_suspend(device_t dev)
  {
          int error;
  
          /* Suspend child Xen devices. */
          error = bus_generic_suspend(dev);
          if (error != 0)
                  return (error);
  
          sx_xlock(&xs.suspend_mutex);
          sx_xlock(&xs.request_mutex);
  
          return (0);
  }
  
  /**
   * Resume XenStore operations after this VM is resumed.
   */
  static int
  xs_resume(device_t dev __unused)
  {
          struct xs_watch *watch;
          char token[sizeof(watch) * 2 + 1];
  
          xs_init_comms();
  
          sx_xunlock(&xs.request_mutex);
  
          /*
           * No need for registered_watches_lock: the suspend_mutex
           * is sufficient.
           */
          LIST_FOREACH(watch, &xs.registered_watches, list) {
                  sprintf(token, "%lX", (long)watch);
                  xs_watch(watch->node, token);
          }
  
          sx_xunlock(&xs.suspend_mutex);
  
          /* Resume child Xen devices. */
          bus_generic_resume(dev);
  
          return (0);
  }
  
  /*-------------------- Private Device Attachment Data  -----------------------*/
  static device_method_t xenstore_methods[] = { 
          /* Device interface */ 
          DEVMETHOD(device_identify,      xs_identify),
          DEVMETHOD(device_probe,         xs_probe), 
          DEVMETHOD(device_attach,        xs_attach), 
          DEVMETHOD(device_detach,        bus_generic_detach), 
          DEVMETHOD(device_shutdown,      bus_generic_shutdown), 
          DEVMETHOD(device_suspend,       xs_suspend), 
          DEVMETHOD(device_resume,        xs_resume), 
   
          /* Bus interface */ 
          DEVMETHOD(bus_add_child,        bus_generic_add_child),
          DEVMETHOD(bus_alloc_resource,   bus_generic_alloc_resource),
          DEVMETHOD(bus_release_resource, bus_generic_release_resource),
          DEVMETHOD(bus_activate_resource, bus_generic_activate_resource),
          DEVMETHOD(bus_deactivate_resource, bus_generic_deactivate_resource),
  
          DEVMETHOD_END
  }; 
  
  DEFINE_CLASS_0(xenstore, xenstore_driver, xenstore_methods, 0);
  static devclass_t xenstore_devclass; 
   
  #ifdef XENHVM
  DRIVER_MODULE(xenstore, xenpci, xenstore_driver, xenstore_devclass, 0, 0);
  #else
  DRIVER_MODULE(xenstore, nexus, xenstore_driver, xenstore_devclass, 0, 0);
  #endif
  
  /*------------------------------- Sysctl Data --------------------------------*/
  /* XXX Shouldn't the node be somewhere else? */
  SYSCTL_NODE(_dev, OID_AUTO, xen, CTLFLAG_RD, NULL, "Xen");
  SYSCTL_INT(_dev_xen, OID_AUTO, xsd_port, CTLFLAG_RD, &xs.evtchn, 0, "");
  SYSCTL_ULONG(_dev_xen, OID_AUTO, xsd_kva, CTLFLAG_RD, (u_long *) &xen_store, 0, "");
  
  /*-------------------------------- Public API --------------------------------*/
  /*------- API comments for these methods can be found in xenstorevar.h -------*/
  int
  xs_directory(struct xs_transaction t, const char *dir, const char *node,
      u_int *num, const char ***result)
  {
          struct sbuf *path;
          char *strings;
          u_int len = 0;
          int error;
  
          path = xs_join(dir, node);
          error = xs_single(t, XS_DIRECTORY, sbuf_data(path), &len,
              (void **)&strings);
          sbuf_delete(path);
          if (error)
                  return (error);
  
          *result = split(strings, len, num);
  
          return (0);
  }
  
  int
  xs_exists(struct xs_transaction t, const char *dir, const char *node)
  {
          const char **d;
          int error, dir_n;
  
          error = xs_directory(t, dir, node, &dir_n, &d);
          if (error)
                  return (0);
          free(d, M_XENSTORE);
          return (1);
  }
  
  int
  xs_read(struct xs_transaction t, const char *dir, const char *node,
      u_int *len, void **result)
  {
          struct sbuf *path;
          void *ret;
          int error;
  
          path = xs_join(dir, node);
          error = xs_single(t, XS_READ, sbuf_data(path), len, &ret);
          sbuf_delete(path);
          if (error)
                  return (error);
          *result = ret;
          return (0);
  }
  
  int
  xs_write(struct xs_transaction t, const char *dir, const char *node,
      const char *string)
  {
          struct sbuf *path;
          struct iovec iovec[2];
          int error;
  
          path = xs_join(dir, node);
  
          iovec[0].iov_base = (void *)(uintptr_t) sbuf_data(path);
          iovec[0].iov_len = sbuf_len(path) + 1;
          iovec[1].iov_base = (void *)(uintptr_t) string;
          iovec[1].iov_len = strlen(string);
  
          error = xs_talkv(t, XS_WRITE, iovec, 2, NULL, NULL);
          sbuf_delete(path);
  
          return (error);
  }
  
  int
  xs_mkdir(struct xs_transaction t, const char *dir, const char *node)
  {
          struct sbuf *path;
          int ret;
  
          path = xs_join(dir, node);
          ret = xs_single(t, XS_MKDIR, sbuf_data(path), NULL, NULL);
          sbuf_delete(path);
  
          return (ret);
  }
  
  int
  xs_rm(struct xs_transaction t, const char *dir, const char *node)
  {
          struct sbuf *path;
          int ret;
  
          path = xs_join(dir, node);
          ret = xs_single(t, XS_RM, sbuf_data(path), NULL, NULL);
          sbuf_delete(path);
  
          return (ret);
  }
  
  int
  xs_rm_tree(struct xs_transaction xbt, const char *base, const char *node)
  {
          struct xs_transaction local_xbt;
          struct sbuf *root_path_sbuf;
          struct sbuf *cur_path_sbuf;
          char *root_path;
          char *cur_path;
          const char **dir;
          int error;
          int empty;
  
  retry:
          root_path_sbuf = xs_join(base, node);
          cur_path_sbuf  = xs_join(base, node);
          root_path      = sbuf_data(root_path_sbuf);
          cur_path       = sbuf_data(cur_path_sbuf);
          dir            = NULL;
          local_xbt.id   = 0;
  
          if (xbt.id == 0) {
                  error = xs_transaction_start(&local_xbt);
                  if (error != 0)
                          goto out;
                  xbt = local_xbt;
          }
  
          empty = 0;
          while (1) {
                  u_int count;
                  u_int i;
  
                  error = xs_directory(xbt, cur_path, "", &count, &dir);
                  if (error)
                          goto out;
  
                  for (i = 0; i < count; i++) {
                          error = xs_rm(xbt, cur_path, dir[i]);
                          if (error == ENOTEMPTY) {
                                  struct sbuf *push_dir;
  
                                  /*
                                   * Descend to clear out this sub directory.
                                   * We'll return to cur_dir once push_dir
                                   * is empty.
                                   */
                                  push_dir = xs_join(cur_path, dir[i]);
                                  sbuf_delete(cur_path_sbuf);
                                  cur_path_sbuf = push_dir;
                                  cur_path = sbuf_data(cur_path_sbuf);
                                  break;
                          } else if (error != 0) {
                                  goto out;
                          }
                  }
  
                  free(dir, M_XENSTORE);
                  dir = NULL;
  
                  if (i == count) {
                          char *last_slash;
  
                          /* Directory is empty.  It is now safe to remove. */
                          error = xs_rm(xbt, cur_path, "");
                          if (error != 0)
                                  goto out;
  
                          if (!strcmp(cur_path, root_path))
                                  break;
  
                          /* Return to processing the parent directory. */
                          last_slash = strrchr(cur_path, '/');
                          KASSERT(last_slash != NULL,
                                  ("xs_rm_tree: mangled path %s", cur_path));
                          *last_slash = '\0';
                  }
          }
  
  out:
          sbuf_delete(cur_path_sbuf);
          sbuf_delete(root_path_sbuf);
          if (dir != NULL)
                  free(dir, M_XENSTORE);
  
          if (local_xbt.id != 0) {
                  int terror;
  
                  terror = xs_transaction_end(local_xbt, /*abort*/error != 0);
                  xbt.id = 0;
                  if (terror == EAGAIN && error == 0)
                          goto retry;
          }
          return (error);
  }
  
  int
  xs_transaction_start(struct xs_transaction *t)
  {
          char *id_str;
          int error;
  
          error = xs_single(XST_NIL, XS_TRANSACTION_START, "", NULL,
              (void **)&id_str);
          if (error == 0) {
                  t->id = strtoul(id_str, NULL, 0);
                  free(id_str, M_XENSTORE);
          }
          return (error);
  }
  
  int
  xs_transaction_end(struct xs_transaction t, int abort)
  {
          char abortstr[2];
  
          if (abort)
                  strcpy(abortstr, "F");
          else
                  strcpy(abortstr, "T");
  
          return (xs_single(t, XS_TRANSACTION_END, abortstr, NULL, NULL));
  }
  
  int
  xs_scanf(struct xs_transaction t, const char *dir, const char *node,
       int *scancountp, const char *fmt, ...)
  {
          va_list ap;
          int error, ns;
          char *val;
  
          error = xs_read(t, dir, node, NULL, (void **) &val);
          if (error)
                  return (error);
  
          va_start(ap, fmt);
          ns = vsscanf(val, fmt, ap);
          va_end(ap);
          free(val, M_XENSTORE);
          /* Distinctive errno. */
          if (ns == 0)
                  return (ERANGE);
          if (scancountp)
                  *scancountp = ns;
          return (0);
  }
  
  int
  xs_vprintf(struct xs_transaction t,
      const char *dir, const char *node, const char *fmt, va_list ap)
  {
          struct sbuf *sb;
          int error;
  
          sb = sbuf_new_auto();
          sbuf_vprintf(sb, fmt, ap);
          sbuf_finish(sb);
          error = xs_write(t, dir, node, sbuf_data(sb));
          sbuf_delete(sb);
  
          return (error);
  }
  
  int
  xs_printf(struct xs_transaction t, const char *dir, const char *node,
       const char *fmt, ...)
  {
          va_list ap;
          int error;
  
          va_start(ap, fmt);
          error = xs_vprintf(t, dir, node, fmt, ap);
          va_end(ap);
  
          return (error);
  }
  
  int
  xs_gather(struct xs_transaction t, const char *dir, ...)
  {
          va_list ap;
          const char *name;
          int error;
  
          va_start(ap, dir);
          error = 0;
          while (error == 0 && (name = va_arg(ap, char *)) != NULL) {
                  const char *fmt = va_arg(ap, char *);
                  void *result = va_arg(ap, void *);
                  char *p;
  
                  error = xs_read(t, dir, name, NULL, (void **) &p);
                  if (error)
                          break;
  
                  if (fmt) {
                          if (sscanf(p, fmt, result) == 0)
                                  error = EINVAL;
                          free(p, M_XENSTORE);
                  } else
                          *(char **)result = p;
          }
          va_end(ap);
  
          return (error);
  }
  
  int
  xs_register_watch(struct xs_watch *watch)
  {
          /* Pointer in ascii is the token. */
          char token[sizeof(watch) * 2 + 1];
          int error;
  
          sprintf(token, "%lX", (long)watch);
  
          sx_slock(&xs.suspend_mutex);
  
          mtx_lock(&xs.registered_watches_lock);
          KASSERT(find_watch(token) == NULL, ("watch already registered"));
          LIST_INSERT_HEAD(&xs.registered_watches, watch, list);
          mtx_unlock(&xs.registered_watches_lock);
  
          error = xs_watch(watch->node, token);
  
          /* Ignore errors due to multiple registration. */
          if (error == EEXIST)
                  error = 0;
  
          if (error != 0) {
                  mtx_lock(&xs.registered_watches_lock);
                  LIST_REMOVE(watch, list);
                  mtx_unlock(&xs.registered_watches_lock);
          }
  
          sx_sunlock(&xs.suspend_mutex);
  
          return (error);
  }
  
  void
  xs_unregister_watch(struct xs_watch *watch)
  {
          struct xs_stored_msg *msg, *tmp;
          char token[sizeof(watch) * 2 + 1];
          int error;
  
          sprintf(token, "%lX", (long)watch);
  
          sx_slock(&xs.suspend_mutex);
  
          mtx_lock(&xs.registered_watches_lock);
          if (find_watch(token) == NULL) {
                  mtx_unlock(&xs.registered_watches_lock);
                  sx_sunlock(&xs.suspend_mutex);
                  return;
          }
          LIST_REMOVE(watch, list);
          mtx_unlock(&xs.registered_watches_lock);
  
          error = xs_unwatch(watch->node, token);
          if (error)
                  log(LOG_WARNING, "XENSTORE Failed to release watch %s: %i\n",
                      watch->node, error);
  
          sx_sunlock(&xs.suspend_mutex);
  
          /* Cancel pending watch events. */
          mtx_lock(&xs.watch_events_lock);
          TAILQ_FOREACH_SAFE(msg, &xs.watch_events, list, tmp) {
                  if (msg->u.watch.handle != watch)
                          continue;
                  TAILQ_REMOVE(&xs.watch_events, msg, list);
                  free(msg->u.watch.vec, M_XENSTORE);
                  free(msg, M_XENSTORE);
          }
          mtx_unlock(&xs.watch_events_lock);
  
          /* Flush any currently-executing callback, unless we are it. :-) */
          if (curproc->p_pid != xs.xenwatch_pid) {
                  sx_xlock(&xs.xenwatch_mutex);
                  sx_xunlock(&xs.xenwatch_mutex);
          }
  }

Cache object: 8e85091addc89f8bbb4b3610815b575f