FreeBSD/Linux Kernel Cross Reference
sys/net/netmap_user.h

     /*
      * Copyright (C) 2011-2014 Universita` di Pisa. All rights reserved.
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
      *
      *   1. Redistributions of source code must retain the above copyright
      *      notice, this list of conditions and the following disclaimer.
     *   2. Redistributions in binary form must reproduce the above copyright
     *      notice, this list of conditions and the following disclaimer in the
     *      documentation and/or other materials provided with the distribution.
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     */
    
    /*
     * $FreeBSD$
     *
     * Functions and macros to manipulate netmap structures and packets
     * in userspace. See netmap(4) for more information.
     *
     * The address of the struct netmap_if, say nifp, is computed from the
     * value returned from ioctl(.., NIOCREG, ...) and the mmap region:
     *      ioctl(fd, NIOCREG, &req);
     *      mem = mmap(0, ... );
     *      nifp = NETMAP_IF(mem, req.nr_nifp);
     *              (so simple, we could just do it manually)
     *
     * From there:
     *      struct netmap_ring *NETMAP_TXRING(nifp, index)
     *      struct netmap_ring *NETMAP_RXRING(nifp, index)
     *              we can access ring->nr_cur, ring->nr_avail, ring->nr_flags
     *
     *      ring->slot[i] gives us the i-th slot (we can access
     *              directly len, flags, buf_idx)
     *
     *      char *buf = NETMAP_BUF(ring, x) returns a pointer to
     *              the buffer numbered x
     *
     * All ring indexes (head, cur, tail) should always move forward.
     * To compute the next index in a circular ring you can use
     *      i = nm_ring_next(ring, i);
     *
     * To ease porting apps from pcap to netmap we supply a few fuctions
     * that can be called to open, close, read and write on netmap in a way
     * similar to libpcap. Note that the read/write function depend on
     * an ioctl()/select()/poll() being issued to refill rings or push
     * packets out.
     *
     * In order to use these, include #define NETMAP_WITH_LIBS
     * in the source file that invokes these functions.
     */
    
    #ifndef _NET_NETMAP_USER_H_
    #define _NET_NETMAP_USER_H_
    
    #include <stdint.h>
    #include <sys/socket.h>         /* apple needs sockaddr */
    #include <net/if.h>             /* IFNAMSIZ */
    
    #ifndef likely
    #define likely(x)       __builtin_expect(!!(x), 1)
    #define unlikely(x)     __builtin_expect(!!(x), 0)
    #endif /* likely and unlikely */
    
    #include <net/netmap.h>
    
    /* helper macro */
    #define _NETMAP_OFFSET(type, ptr, offset) \
            ((type)(void *)((char *)(ptr) + (offset)))
    
    #define NETMAP_IF(_base, _ofs)  _NETMAP_OFFSET(struct netmap_if *, _base, _ofs)
    
    #define NETMAP_TXRING(nifp, index) _NETMAP_OFFSET(struct netmap_ring *, \
            nifp, (nifp)->ring_ofs[index] )
    
    #define NETMAP_RXRING(nifp, index) _NETMAP_OFFSET(struct netmap_ring *, \
            nifp, (nifp)->ring_ofs[index + (nifp)->ni_tx_rings + 1] )
    
    #define NETMAP_BUF(ring, index)                         \
            ((char *)(ring) + (ring)->buf_ofs + ((index)*(ring)->nr_buf_size))
    
    #define NETMAP_BUF_IDX(ring, buf)                       \
            ( ((char *)(buf) - ((char *)(ring) + (ring)->buf_ofs) ) / \
                    (ring)->nr_buf_size )
    
    
    static inline uint32_t
   nm_ring_next(struct netmap_ring *r, uint32_t i)
   {
           return ( unlikely(i + 1 == r->num_slots) ? 0 : i + 1);
   }
   
   
   /*
    * Return 1 if we have pending transmissions in the tx ring.
    * When everything is complete ring->head = ring->tail + 1 (modulo ring size)
    */
   static inline int
   nm_tx_pending(struct netmap_ring *r)
   {
           return nm_ring_next(r, r->tail) != r->head;
   }
   
   
   static inline uint32_t
   nm_ring_space(struct netmap_ring *ring)
   {
           int ret = ring->tail - ring->cur;
           if (ret < 0)
                   ret += ring->num_slots;
           return ret;
   }
   
   
   #ifdef NETMAP_WITH_LIBS
   /*
    * Support for simple I/O libraries.
    * Include other system headers required for compiling this.
    */
   
   #ifndef HAVE_NETMAP_WITH_LIBS
   #define HAVE_NETMAP_WITH_LIBS
   
   #include <sys/time.h>
   #include <sys/mman.h>
   #include <string.h>     /* memset */
   #include <sys/ioctl.h>
   #include <sys/errno.h>  /* EINVAL */
   #include <fcntl.h>      /* O_RDWR */
   #include <unistd.h>     /* close() */
   #include <signal.h>
   #include <stdlib.h>
   
   #ifndef ND /* debug macros */
   /* debug support */
   #define ND(_fmt, ...) do {} while(0)
   #define D(_fmt, ...)                                            \
           do {                                                    \
                   struct timeval t0;                              \
                   gettimeofday(&t0, NULL);                        \
                   fprintf(stderr, "%03d.%06d %s [%d] " _fmt "\n", \
                       (int)(t0.tv_sec % 1000), (int)t0.tv_usec,   \
                       __FUNCTION__, __LINE__, ##__VA_ARGS__);     \
           } while (0)
   
   /* Rate limited version of "D", lps indicates how many per second */
   #define RD(lps, format, ...)                                    \
       do {                                                        \
           static int t0, __cnt;                                   \
           struct timeval __xxts;                                  \
           gettimeofday(&__xxts, NULL);                            \
           if (t0 != __xxts.tv_sec) {                              \
               t0 = __xxts.tv_sec;                                 \
               __cnt = 0;                                          \
           }                                                       \
           if (__cnt++ < lps) {                                    \
               D(format, ##__VA_ARGS__);                           \
           }                                                       \
       } while (0)
   #endif
   
   struct nm_pkthdr {      /* same as pcap_pkthdr */
           struct timeval  ts;
           uint32_t        caplen;
           uint32_t        len;
   };
   
   struct nm_stat {        /* same as pcap_stat    */
           u_int   ps_recv;
           u_int   ps_drop;
           u_int   ps_ifdrop;
   #ifdef WIN32
           u_int   bs_capt;
   #endif /* WIN32 */
   };
   
   #define NM_ERRBUF_SIZE  512
   
   struct nm_desc {
           struct nm_desc *self; /* point to self if netmap. */
           int fd;
           void *mem;
           int memsize;
           int done_mmap;  /* set if mem is the result of mmap */
           struct netmap_if * const nifp;
           uint16_t first_tx_ring, last_tx_ring, cur_tx_ring;
           uint16_t first_rx_ring, last_rx_ring, cur_rx_ring;
           struct nmreq req;       /* also contains the nr_name = ifname */
           struct nm_pkthdr hdr;
   
           /*
            * The memory contains netmap_if, rings and then buffers.
            * Given a pointer (e.g. to nm_inject) we can compare with
            * mem/buf_start/buf_end to tell if it is a buffer or
            * some other descriptor in our region.
            * We also store a pointer to some ring as it helps in the
            * translation from buffer indexes to addresses.
            */
           struct netmap_ring * const some_ring;
           void * const buf_start;
           void * const buf_end;
           /* parameters from pcap_open_live */
           int snaplen;
           int promisc;
           int to_ms;
           char *errbuf;
   
           /* save flags so we can restore them on close */
           uint32_t if_flags;
           uint32_t if_reqcap;
           uint32_t if_curcap;
   
           struct nm_stat st;
           char msg[NM_ERRBUF_SIZE];
   };
   
   /*
    * when the descriptor is open correctly, d->self == d
    * Eventually we should also use some magic number.
    */
   #define P2NMD(p)                ((struct nm_desc *)(p))
   #define IS_NETMAP_DESC(d)       ((d) && P2NMD(d)->self == P2NMD(d))
   #define NETMAP_FD(d)            (P2NMD(d)->fd)
   
   
   /*
    * this is a slightly optimized copy routine which rounds
    * to multiple of 64 bytes and is often faster than dealing
    * with other odd sizes. We assume there is enough room
    * in the source and destination buffers.
    *
    * XXX only for multiples of 64 bytes, non overlapped.
    */
   static inline void
   nm_pkt_copy(const void *_src, void *_dst, int l)
   {
           const uint64_t *src = (const uint64_t *)_src;
           uint64_t *dst = (uint64_t *)_dst;
   
           if (unlikely(l >= 1024)) {
                   memcpy(dst, src, l);
                   return;
           }
           for (; likely(l > 0); l-=64) {
                   *dst++ = *src++;
                   *dst++ = *src++;
                   *dst++ = *src++;
                   *dst++ = *src++;
                   *dst++ = *src++;
                   *dst++ = *src++;
                   *dst++ = *src++;
                   *dst++ = *src++;
           }
   }
   
   
   /*
    * The callback, invoked on each received packet. Same as libpcap
    */
   typedef void (*nm_cb_t)(u_char *, const struct nm_pkthdr *, const u_char *d);
   
   /*
    *--- the pcap-like API ---
    *
    * nm_open() opens a file descriptor, binds to a port and maps memory.
    *
    * ifname       (netmap:foo or vale:foo) is the port name
    *              a suffix can indicate the follwing:
    *              ^               bind the host (sw) ring pair
    *              *               bind host and NIC ring pairs (transparent)
    *              -NN             bind individual NIC ring pair
    *              {NN             bind master side of pipe NN
    *              }NN             bind slave side of pipe NN
    *
    * req          provides the initial values of nmreq before parsing ifname.
    *              Remember that the ifname parsing will override the ring
    *              number in nm_ringid, and part of nm_flags;
    * flags        special functions, normally 0
    *              indicates which fields of *arg are significant
    * arg          special functions, normally NULL
    *              if passed a netmap_desc with mem != NULL,
    *              use that memory instead of mmap.
    */
    
   static struct nm_desc *nm_open(const char *ifname, const struct nmreq *req,
           uint64_t flags, const struct nm_desc *arg);
   
   /*
    * nm_open can import some fields from the parent descriptor.
    * These flags control which ones.
    * Also in flags you can specify NETMAP_NO_TX_POLL and NETMAP_DO_RX_POLL,
    * which set the initial value for these flags.
    * Note that the 16 low bits of the flags are reserved for data
    * that may go into the nmreq.
    */
   enum {
           NM_OPEN_NO_MMAP =       0x040000, /* reuse mmap from parent */
           NM_OPEN_IFNAME =        0x080000, /* nr_name, nr_ringid, nr_flags */
           NM_OPEN_ARG1 =          0x100000,
           NM_OPEN_ARG2 =          0x200000,
           NM_OPEN_ARG3 =          0x400000,
           NM_OPEN_RING_CFG =      0x800000, /* tx|rx rings|slots */
   };
   
   
   /*
    * nm_close()   closes and restores the port to its previous state
    */
   
   static int nm_close(struct nm_desc *);
   
   /*
    * nm_inject() is the same as pcap_inject()
    * nm_dispatch() is the same as pcap_dispatch()
    * nm_nextpkt() is the same as pcap_next()
    */
   
   static int nm_inject(struct nm_desc *, const void *, size_t);
   static int nm_dispatch(struct nm_desc *, int, nm_cb_t, u_char *);
   static u_char *nm_nextpkt(struct nm_desc *, struct nm_pkthdr *);
   
   
   /*
    * Try to open, return descriptor if successful, NULL otherwise.
    * An invalid netmap name will return errno = 0;
    * You can pass a pointer to a pre-filled nm_desc to add special
    * parameters. Flags is used as follows
    * NM_OPEN_NO_MMAP      use the memory from arg, only
    *                      if the nr_arg2 (memory block) matches.
    * NM_OPEN_ARG1         use req.nr_arg1 from arg
    * NM_OPEN_ARG2         use req.nr_arg2 from arg
    * NM_OPEN_RING_CFG     user ring config from arg
    */
   static struct nm_desc *
   nm_open(const char *ifname, const struct nmreq *req,
           uint64_t new_flags, const struct nm_desc *arg)
   {
           struct nm_desc *d = NULL;
           const struct nm_desc *parent = arg;
           u_int namelen;
           uint32_t nr_ringid = 0, nr_flags;
           const char *port = NULL;
           const char *errmsg = NULL;
   
           if (strncmp(ifname, "netmap:", 7) && strncmp(ifname, "vale", 4)) {
                   errno = 0; /* name not recognised, not an error */
                   return NULL;
           }
           if (ifname[0] == 'n')
                   ifname += 7;
           /* scan for a separator */
           for (port = ifname; *port && !index("-*^{}", *port); port++)
                   ;
           namelen = port - ifname;
           if (namelen >= sizeof(d->req.nr_name)) {
                   errmsg = "name too long";
                   goto fail;
           }
           switch (*port) {
           default:  /* '\0', no suffix */
                   nr_flags = NR_REG_ALL_NIC;
                   break;
           case '-': /* one NIC */
                   nr_flags = NR_REG_ONE_NIC;
                   nr_ringid = atoi(port + 1);
                   break;
           case '*': /* NIC and SW, ignore port */
                   nr_flags = NR_REG_NIC_SW;
                   if (port[1]) {
                           errmsg = "invalid port for nic+sw";
                           goto fail;
                   }
                   break;
           case '^': /* only sw ring */
                   nr_flags = NR_REG_SW;
                   if (port[1]) {
                           errmsg = "invalid port for sw ring";
                           goto fail;
                   }
                   break;
           case '{':
                   nr_flags = NR_REG_PIPE_MASTER;
                   nr_ringid = atoi(port + 1);
                   break;
           case '}':
                   nr_flags = NR_REG_PIPE_SLAVE;
                   nr_ringid = atoi(port + 1);
                   break;
           }
   
           if (nr_ringid >= NETMAP_RING_MASK) {
                   errmsg = "invalid ringid";
                   goto fail;
           }
           /* add the *XPOLL flags */
           nr_ringid |= new_flags & (NETMAP_NO_TX_POLL | NETMAP_DO_RX_POLL);
   
           d = (struct nm_desc *)calloc(1, sizeof(*d));
           if (d == NULL) {
                   errmsg = "nm_desc alloc failure";
                   errno = ENOMEM;
                   return NULL;
           }
           d->self = d;    /* set this early so nm_close() works */
           d->fd = open("/dev/netmap", O_RDWR);
           if (d->fd < 0) {
                   errmsg = "cannot open /dev/netmap";
                   goto fail;
           }
   
           if (req)
                   d->req = *req;
           d->req.nr_version = NETMAP_API;
           d->req.nr_ringid &= ~NETMAP_RING_MASK;
   
           /* these fields are overridden by ifname and flags processing */
           d->req.nr_ringid |= nr_ringid;
           d->req.nr_flags = nr_flags;
           memcpy(d->req.nr_name, ifname, namelen);
           d->req.nr_name[namelen] = '\0';
           /* optionally import info from parent */
           if (IS_NETMAP_DESC(parent) && new_flags) {
                   if (new_flags & NM_OPEN_ARG1)
                           D("overriding ARG1 %d", parent->req.nr_arg1);
                   d->req.nr_arg1 = new_flags & NM_OPEN_ARG1 ?
                           parent->req.nr_arg1 : 4;
                   if (new_flags & NM_OPEN_ARG2)
                           D("overriding ARG2 %d", parent->req.nr_arg2);
                   d->req.nr_arg2 = new_flags & NM_OPEN_ARG2 ?
                           parent->req.nr_arg2 : 0;
                   if (new_flags & NM_OPEN_ARG3)
                           D("overriding ARG3 %d", parent->req.nr_arg3);
                   d->req.nr_arg3 = new_flags & NM_OPEN_ARG3 ?
                           parent->req.nr_arg3 : 0;
                   if (new_flags & NM_OPEN_RING_CFG) {
                           D("overriding RING_CFG");
                           d->req.nr_tx_slots = parent->req.nr_tx_slots;
                           d->req.nr_rx_slots = parent->req.nr_rx_slots;
                           d->req.nr_tx_rings = parent->req.nr_tx_rings;
                           d->req.nr_rx_rings = parent->req.nr_rx_rings;
                   }
                   if (new_flags & NM_OPEN_IFNAME) {
                           D("overriding ifname %s ringid 0x%x flags 0x%x",
                                   parent->req.nr_name, parent->req.nr_ringid,
                                   parent->req.nr_flags);
                           memcpy(d->req.nr_name, parent->req.nr_name,
                                   sizeof(d->req.nr_name));
                           d->req.nr_ringid = parent->req.nr_ringid;
                           d->req.nr_flags = parent->req.nr_flags;
                   }
           }
           if (ioctl(d->fd, NIOCREGIF, &d->req)) {
                   errmsg = "NIOCREGIF failed";
                   goto fail;
           }
   
           if (IS_NETMAP_DESC(parent) && parent->mem &&
               parent->req.nr_arg2 == d->req.nr_arg2) {
                   /* do not mmap, inherit from parent */
                   d->memsize = parent->memsize;
                   d->mem = parent->mem;
           } else {
                   d->memsize = d->req.nr_memsize;
                   d->mem = mmap(0, d->memsize, PROT_WRITE | PROT_READ, MAP_SHARED,
                                   d->fd, 0);
                   if (d->mem == NULL) {
                           errmsg = "mmap failed";
                           goto fail;
                   }
                   d->done_mmap = 1;
           }
           {
                   struct netmap_if *nifp = NETMAP_IF(d->mem, d->req.nr_offset);
                   struct netmap_ring *r = NETMAP_RXRING(nifp, );
   
                   *(struct netmap_if **)(uintptr_t)&(d->nifp) = nifp;
                   *(struct netmap_ring **)(uintptr_t)&d->some_ring = r;
                   *(void **)(uintptr_t)&d->buf_start = NETMAP_BUF(r, 0);
                   *(void **)(uintptr_t)&d->buf_end =
                           (char *)d->mem + d->memsize;
           }
   
           if (nr_flags ==  NR_REG_SW) { /* host stack */
                   d->first_tx_ring = d->last_tx_ring = d->req.nr_tx_rings;
                   d->first_rx_ring = d->last_rx_ring = d->req.nr_rx_rings;
           } else if (nr_flags ==  NR_REG_ALL_NIC) { /* only nic */
                   d->first_tx_ring = 0;
                   d->first_rx_ring = 0;
                   d->last_tx_ring = d->req.nr_tx_rings - 1;
                   d->last_rx_ring = d->req.nr_rx_rings - 1;
           } else if (nr_flags ==  NR_REG_NIC_SW) {
                   d->first_tx_ring = 0;
                   d->first_rx_ring = 0;
                   d->last_tx_ring = d->req.nr_tx_rings;
                   d->last_rx_ring = d->req.nr_rx_rings;
           } else if (nr_flags == NR_REG_ONE_NIC) {
                   /* XXX check validity */
                   d->first_tx_ring = d->last_tx_ring =
                   d->first_rx_ring = d->last_rx_ring = nr_ringid;
           } else { /* pipes */
                   d->first_tx_ring = d->last_tx_ring = 0;
                   d->first_rx_ring = d->last_rx_ring = 0;
           }
   
   #ifdef DEBUG_NETMAP_USER
       { /* debugging code */
           int i;
   
           D("%s tx %d .. %d %d rx %d .. %d %d", ifname,
                   d->first_tx_ring, d->last_tx_ring, d->req.nr_tx_rings,
                   d->first_rx_ring, d->last_rx_ring, d->req.nr_rx_rings);
           for (i = 0; i <= d->req.nr_tx_rings; i++) {
                   struct netmap_ring *r = NETMAP_TXRING(d->nifp, i);
                   D("TX%d %p h %d c %d t %d", i, r, r->head, r->cur, r->tail);
           }
           for (i = 0; i <= d->req.nr_rx_rings; i++) {
                   struct netmap_ring *r = NETMAP_RXRING(d->nifp, i);
                   D("RX%d %p h %d c %d t %d", i, r, r->head, r->cur, r->tail);
           }
       }
   #endif /* debugging */
                   
           d->cur_tx_ring = d->first_tx_ring;
           d->cur_rx_ring = d->first_rx_ring;
           return d;
   
   fail:
           nm_close(d);
           if (errmsg)
                   D("%s %s", errmsg, ifname);
           errno = EINVAL;
           return NULL;
   }
   
   
   static int
   nm_close(struct nm_desc *d)
   {
           /*
            * ugly trick to avoid unused warnings
            */
           static void *__xxzt[] __attribute__ ((unused))  =
                   { (void *)nm_open, (void *)nm_inject,
                     (void *)nm_dispatch, (void *)nm_nextpkt } ;
   
           if (d == NULL || d->self != d)
                   return EINVAL;
           if (d->done_mmap && d->mem)
                   munmap(d->mem, d->memsize);
           if (d->fd != -1)
                   close(d->fd);
           bzero(d, sizeof(*d));
           free(d);
           return 0;
   }
   
   
   /*
    * Same prototype as pcap_inject(), only need to cast.
    */
   static int
   nm_inject(struct nm_desc *d, const void *buf, size_t size)
   {
           u_int c, n = d->last_tx_ring - d->first_tx_ring + 1;
   
           for (c = 0; c < n ; c++) {
                   /* compute current ring to use */
                   struct netmap_ring *ring;
                   uint32_t i, idx;
                   uint32_t ri = d->cur_tx_ring + c;
   
                   if (ri > d->last_tx_ring)
                           ri = d->first_tx_ring;
                   ring = NETMAP_TXRING(d->nifp, ri);
                   if (nm_ring_empty(ring)) {
                           continue;
                   }
                   i = ring->cur;
                   idx = ring->slot[i].buf_idx;
                   ring->slot[i].len = size;
                   nm_pkt_copy(buf, NETMAP_BUF(ring, idx), size);
                   d->cur_tx_ring = ri;
                   ring->head = ring->cur = nm_ring_next(ring, i);
                   return size;
           }
           return 0; /* fail */
   }
   
   
   /*
    * Same prototype as pcap_dispatch(), only need to cast.
    */
   static int
   nm_dispatch(struct nm_desc *d, int cnt, nm_cb_t cb, u_char *arg)
   {
           int n = d->last_rx_ring - d->first_rx_ring + 1;
           int c, got = 0, ri = d->cur_rx_ring;
   
           if (cnt == 0)
                   cnt = -1;
           /* cnt == -1 means infinite, but rings have a finite amount
            * of buffers and the int is large enough that we never wrap,
            * so we can omit checking for -1
            */
           for (c=0; c < n && cnt != got; c++) {
                   /* compute current ring to use */
                   struct netmap_ring *ring;
   
                   ri = d->cur_rx_ring + c;
                   if (ri > d->last_rx_ring)
                           ri = d->first_rx_ring;
                   ring = NETMAP_RXRING(d->nifp, ri);
                   for ( ; !nm_ring_empty(ring) && cnt != got; got++) {
                           u_int i = ring->cur;
                           u_int idx = ring->slot[i].buf_idx;
                           u_char *buf = (u_char *)NETMAP_BUF(ring, idx);
   
                           // __builtin_prefetch(buf);
                           d->hdr.len = d->hdr.caplen = ring->slot[i].len;
                           d->hdr.ts = ring->ts;
                           cb(arg, &d->hdr, buf);
                           ring->head = ring->cur = nm_ring_next(ring, i);
                   }
           }
           d->cur_rx_ring = ri;
           return got;
   }
   
   static u_char *
   nm_nextpkt(struct nm_desc *d, struct nm_pkthdr *hdr)
   {
           int ri = d->cur_rx_ring;
   
           do {
                   /* compute current ring to use */
                   struct netmap_ring *ring = NETMAP_RXRING(d->nifp, ri);
                   if (!nm_ring_empty(ring)) {
                           u_int i = ring->cur;
                           u_int idx = ring->slot[i].buf_idx;
                           u_char *buf = (u_char *)NETMAP_BUF(ring, idx);
   
                           // __builtin_prefetch(buf);
                           hdr->ts = ring->ts;
                           hdr->len = hdr->caplen = ring->slot[i].len;
                           ring->cur = nm_ring_next(ring, i);
                           /* we could postpone advancing head if we want
                            * to hold the buffer. This can be supported in
                            * the future.
                            */
                           ring->head = ring->cur;
                           d->cur_rx_ring = ri;
                           return buf;
                   }
                   ri++;
                   if (ri > d->last_rx_ring)
                           ri = d->first_rx_ring;
           } while (ri != d->cur_rx_ring);
           return NULL; /* nothing found */
   }
   
   #endif /* !HAVE_NETMAP_WITH_LIBS */
   
   #endif /* NETMAP_WITH_LIBS */
   
   #endif /* _NET_NETMAP_USER_H_ */

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