FreeBSD/Linux Kernel Cross Reference
sys/i386/i386/vm_machdep.c

     /*-
      * SPDX-License-Identifier: BSD-4-Clause
      *
      * Copyright (c) 1982, 1986 The Regents of the University of California.
      * Copyright (c) 1989, 1990 William Jolitz
      * Copyright (c) 1994 John Dyson
      * All rights reserved.
      *
      * This code is derived from software contributed to Berkeley by
     * the Systems Programming Group of the University of Utah Computer
     * Science Department, and William Jolitz.
     *
     * 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.
     * 3. All advertising materials mentioning features or use of this software
     *    must display the following acknowledgement:
     *      This product includes software developed by the University of
     *      California, Berkeley and its contributors.
     * 4. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
     *
     *      from: @(#)vm_machdep.c  7.3 (Berkeley) 5/13/91
     *      Utah $Hdr: vm_machdep.c 1.16.1.1 89/06/23$
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD: releng/12.0/sys/i386/i386/vm_machdep.c 338807 2018-09-19 19:35:02Z kib $");
    
    #include "opt_isa.h"
    #include "opt_npx.h"
    #include "opt_reset.h"
    #include "opt_cpu.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/bio.h>
    #include <sys/buf.h>
    #include <sys/kernel.h>
    #include <sys/ktr.h>
    #include <sys/lock.h>
    #include <sys/malloc.h>
    #include <sys/mbuf.h>
    #include <sys/mutex.h>
    #include <sys/pioctl.h>
    #include <sys/proc.h>
    #include <sys/sysent.h>
    #include <sys/sf_buf.h>
    #include <sys/smp.h>
    #include <sys/sched.h>
    #include <sys/sysctl.h>
    #include <sys/unistd.h>
    #include <sys/vnode.h>
    #include <sys/vmmeter.h>
    
    #include <machine/cpu.h>
    #include <machine/cputypes.h>
    #include <machine/md_var.h>
    #include <machine/pcb.h>
    #include <machine/pcb_ext.h>
    #include <machine/smp.h>
    #include <machine/vm86.h>
    
    #include <vm/vm.h>
    #include <vm/vm_extern.h>
    #include <vm/vm_kern.h>
    #include <vm/vm_page.h>
    #include <vm/vm_map.h>
    #include <vm/vm_param.h>
    
    #ifndef NSFBUFS
    #define NSFBUFS         (512 + maxusers * 16)
    #endif
    
    _Static_assert(OFFSETOF_CURTHREAD == offsetof(struct pcpu, pc_curthread),
        "OFFSETOF_CURTHREAD does not correspond with offset of pc_curthread.");
    _Static_assert(OFFSETOF_CURPCB == offsetof(struct pcpu, pc_curpcb),
        "OFFSETOF_CURPCB does not correspond with offset of pc_curpcb.");
    _Static_assert(__OFFSETOF_MONITORBUF == offsetof(struct pcpu, pc_monitorbuf),
        "__OFFSETOF_MONINORBUF does not correspond with offset of pc_monitorbuf.");
    
   union savefpu *
   get_pcb_user_save_td(struct thread *td)
   {
           vm_offset_t p;
   
           p = td->td_kstack + td->td_kstack_pages * PAGE_SIZE -
               roundup2(cpu_max_ext_state_size, XSAVE_AREA_ALIGN);
           KASSERT((p % XSAVE_AREA_ALIGN) == 0, ("Unaligned pcb_user_save area"));
           return ((union savefpu *)p);
   }
   
   union savefpu *
   get_pcb_user_save_pcb(struct pcb *pcb)
   {
           vm_offset_t p;
   
           p = (vm_offset_t)(pcb + 1);
           return ((union savefpu *)p);
   }
   
   struct pcb *
   get_pcb_td(struct thread *td)
   {
           vm_offset_t p;
   
           p = td->td_kstack + td->td_kstack_pages * PAGE_SIZE -
               roundup2(cpu_max_ext_state_size, XSAVE_AREA_ALIGN) -
               sizeof(struct pcb);
           return ((struct pcb *)p);
   }
   
   void *
   alloc_fpusave(int flags)
   {
           void *res;
           struct savefpu_ymm *sf;
   
           res = malloc(cpu_max_ext_state_size, M_DEVBUF, flags);
           if (use_xsave) {
                   sf = (struct savefpu_ymm *)res;
                   bzero(&sf->sv_xstate.sx_hd, sizeof(sf->sv_xstate.sx_hd));
                   sf->sv_xstate.sx_hd.xstate_bv = xsave_mask;
           }
           return (res);
   }
   /*
    * Finish a fork operation, with process p2 nearly set up.
    * Copy and update the pcb, set up the stack so that the child
    * ready to run and return to user mode.
    */
   void
   cpu_fork(struct thread *td1, struct proc *p2, struct thread *td2, int flags)
   {
           struct proc *p1;
           struct pcb *pcb2;
           struct mdproc *mdp2;
   
           p1 = td1->td_proc;
           if ((flags & RFPROC) == 0) {
                   if ((flags & RFMEM) == 0) {
                           /* unshare user LDT */
                           struct mdproc *mdp1 = &p1->p_md;
                           struct proc_ldt *pldt, *pldt1;
   
                           mtx_lock_spin(&dt_lock);
                           if ((pldt1 = mdp1->md_ldt) != NULL &&
                               pldt1->ldt_refcnt > 1) {
                                   pldt = user_ldt_alloc(mdp1, pldt1->ldt_len);
                                   if (pldt == NULL)
                                           panic("could not copy LDT");
                                   mdp1->md_ldt = pldt;
                                   set_user_ldt(mdp1);
                                   user_ldt_deref(pldt1);
                           } else
                                   mtx_unlock_spin(&dt_lock);
                   }
                   return;
           }
   
           /* Ensure that td1's pcb is up to date. */
           if (td1 == curthread)
                   td1->td_pcb->pcb_gs = rgs();
           critical_enter();
           if (PCPU_GET(fpcurthread) == td1)
                   npxsave(td1->td_pcb->pcb_save);
           critical_exit();
   
           /* Point the pcb to the top of the stack */
           pcb2 = get_pcb_td(td2);
           td2->td_pcb = pcb2;
   
           /* Copy td1's pcb */
           bcopy(td1->td_pcb, pcb2, sizeof(*pcb2));
   
           /* Properly initialize pcb_save */
           pcb2->pcb_save = get_pcb_user_save_pcb(pcb2);
           bcopy(get_pcb_user_save_td(td1), get_pcb_user_save_pcb(pcb2),
               cpu_max_ext_state_size);
   
           /* Point mdproc and then copy over td1's contents */
           mdp2 = &p2->p_md;
           bcopy(&p1->p_md, mdp2, sizeof(*mdp2));
   
           /*
            * Create a new fresh stack for the new process.
            * Copy the trap frame for the return to user mode as if from a
            * syscall.  This copies most of the user mode register values.
            * The -VM86_STACK_SPACE (-16) is so we can expand the trapframe
            * if we go to vm86.
            */
           td2->td_frame = (struct trapframe *)((caddr_t)td2->td_pcb -
               VM86_STACK_SPACE) - 1;
           bcopy(td1->td_frame, td2->td_frame, sizeof(struct trapframe));
   
           td2->td_frame->tf_eax = 0;              /* Child returns zero */
           td2->td_frame->tf_eflags &= ~PSL_C;     /* success */
           td2->td_frame->tf_edx = 1;
   
           /*
            * If the parent process has the trap bit set (i.e. a debugger had
            * single stepped the process to the system call), we need to clear
            * the trap flag from the new frame unless the debugger had set PF_FORK
            * on the parent.  Otherwise, the child will receive a (likely
            * unexpected) SIGTRAP when it executes the first instruction after
            * returning  to userland.
            */
           if ((p1->p_pfsflags & PF_FORK) == 0)
                   td2->td_frame->tf_eflags &= ~PSL_T;
   
           /*
            * Set registers for trampoline to user mode.  Leave space for the
            * return address on stack.  These are the kernel mode register values.
            */
   #if defined(PAE) || defined(PAE_TABLES)
           pcb2->pcb_cr3 = vtophys(vmspace_pmap(p2->p_vmspace)->pm_pdpt);
   #else
           pcb2->pcb_cr3 = vtophys(vmspace_pmap(p2->p_vmspace)->pm_pdir);
   #endif
           pcb2->pcb_edi = 0;
           pcb2->pcb_esi = (int)fork_return;       /* fork_trampoline argument */
           pcb2->pcb_ebp = 0;
           pcb2->pcb_esp = (int)td2->td_frame - sizeof(void *);
           pcb2->pcb_ebx = (int)td2;               /* fork_trampoline argument */
           pcb2->pcb_eip = (int)fork_trampoline + setidt_disp;
           /*-
            * pcb2->pcb_dr*:       cloned above.
            * pcb2->pcb_savefpu:   cloned above.
            * pcb2->pcb_flags:     cloned above.
            * pcb2->pcb_onfault:   cloned above (always NULL here?).
            * pcb2->pcb_gs:        cloned above.
            * pcb2->pcb_ext:       cleared below.
            */
   
           /*
            * XXX don't copy the i/o pages.  this should probably be fixed.
            */
           pcb2->pcb_ext = 0;
   
           /* Copy the LDT, if necessary. */
           mtx_lock_spin(&dt_lock);
           if (mdp2->md_ldt != NULL) {
                   if (flags & RFMEM) {
                           mdp2->md_ldt->ldt_refcnt++;
                   } else {
                           mdp2->md_ldt = user_ldt_alloc(mdp2,
                               mdp2->md_ldt->ldt_len);
                           if (mdp2->md_ldt == NULL)
                                   panic("could not copy LDT");
                   }
           }
           mtx_unlock_spin(&dt_lock);
   
           /* Setup to release spin count in fork_exit(). */
           td2->td_md.md_spinlock_count = 1;
           td2->td_md.md_saved_flags = PSL_KERNEL | PSL_I;
   
           /*
            * Now, cpu_switch() can schedule the new process.
            * pcb_esp is loaded pointing to the cpu_switch() stack frame
            * containing the return address when exiting cpu_switch.
            * This will normally be to fork_trampoline(), which will have
            * %ebx loaded with the new proc's pointer.  fork_trampoline()
            * will set up a stack to call fork_return(p, frame); to complete
            * the return to user-mode.
            */
   }
   
   /*
    * Intercept the return address from a freshly forked process that has NOT
    * been scheduled yet.
    *
    * This is needed to make kernel threads stay in kernel mode.
    */
   void
   cpu_fork_kthread_handler(struct thread *td, void (*func)(void *), void *arg)
   {
           /*
            * Note that the trap frame follows the args, so the function
            * is really called like this:  func(arg, frame);
            */
           td->td_pcb->pcb_esi = (int) func;       /* function */
           td->td_pcb->pcb_ebx = (int) arg;        /* first arg */
   }
   
   void
   cpu_exit(struct thread *td)
   {
   
           /*
            * If this process has a custom LDT, release it.  Reset pc->pcb_gs
            * and %gs before we free it in case they refer to an LDT entry.
            */
           mtx_lock_spin(&dt_lock);
           if (td->td_proc->p_md.md_ldt) {
                   td->td_pcb->pcb_gs = _udatasel;
                   load_gs(_udatasel);
                   user_ldt_free(td);
           } else
                   mtx_unlock_spin(&dt_lock);
   }
   
   void
   cpu_thread_exit(struct thread *td)
   {
   
           critical_enter();
           if (td == PCPU_GET(fpcurthread))
                   npxdrop();
           critical_exit();
   
           /* Disable any hardware breakpoints. */
           if (td->td_pcb->pcb_flags & PCB_DBREGS) {
                   reset_dbregs();
                   td->td_pcb->pcb_flags &= ~PCB_DBREGS;
           }
   }
   
   void
   cpu_thread_clean(struct thread *td)
   {
           struct pcb *pcb;
   
           pcb = td->td_pcb; 
           if (pcb->pcb_ext != NULL) {
                   /* if (pcb->pcb_ext->ext_refcount-- == 1) ?? */
                   /*
                    * XXX do we need to move the TSS off the allocated pages
                    * before freeing them?  (not done here)
                    */
                   pmap_trm_free(pcb->pcb_ext, ctob(IOPAGES + 1));
                   pcb->pcb_ext = NULL;
           }
   }
   
   void
   cpu_thread_swapin(struct thread *td)
   {
   }
   
   void
   cpu_thread_swapout(struct thread *td)
   {
   }
   
   void
   cpu_thread_alloc(struct thread *td)
   {
           struct pcb *pcb;
           struct xstate_hdr *xhdr;
   
           td->td_pcb = pcb = get_pcb_td(td);
           td->td_frame = (struct trapframe *)((caddr_t)pcb -
               VM86_STACK_SPACE) - 1;
           pcb->pcb_ext = NULL; 
           pcb->pcb_save = get_pcb_user_save_pcb(pcb);
           if (use_xsave) {
                   xhdr = (struct xstate_hdr *)(pcb->pcb_save + 1);
                   bzero(xhdr, sizeof(*xhdr));
                   xhdr->xstate_bv = xsave_mask;
           }
   }
   
   void
   cpu_thread_free(struct thread *td)
   {
   
           cpu_thread_clean(td);
   }
   
   void
   cpu_set_syscall_retval(struct thread *td, int error)
   {
   
           switch (error) {
           case 0:
                   td->td_frame->tf_eax = td->td_retval[0];
                   td->td_frame->tf_edx = td->td_retval[1];
                   td->td_frame->tf_eflags &= ~PSL_C;
                   break;
   
           case ERESTART:
                   /*
                    * Reconstruct pc, assuming lcall $X,y is 7 bytes, int
                    * 0x80 is 2 bytes. We saved this in tf_err.
                    */
                   td->td_frame->tf_eip -= td->td_frame->tf_err;
                   break;
   
           case EJUSTRETURN:
                   break;
   
           default:
                   td->td_frame->tf_eax = SV_ABI_ERRNO(td->td_proc, error);
                   td->td_frame->tf_eflags |= PSL_C;
                   break;
           }
   }
   
   /*
    * Initialize machine state, mostly pcb and trap frame for a new
    * thread, about to return to userspace.  Put enough state in the new
    * thread's PCB to get it to go back to the fork_return(), which
    * finalizes the thread state and handles peculiarities of the first
    * return to userspace for the new thread.
    */
   void
   cpu_copy_thread(struct thread *td, struct thread *td0)
   {
           struct pcb *pcb2;
   
           /* Point the pcb to the top of the stack. */
           pcb2 = td->td_pcb;
   
           /*
            * Copy the upcall pcb.  This loads kernel regs.
            * Those not loaded individually below get their default
            * values here.
            */
           bcopy(td0->td_pcb, pcb2, sizeof(*pcb2));
           pcb2->pcb_flags &= ~(PCB_NPXINITDONE | PCB_NPXUSERINITDONE |
               PCB_KERNNPX);
           pcb2->pcb_save = get_pcb_user_save_pcb(pcb2);
           bcopy(get_pcb_user_save_td(td0), pcb2->pcb_save,
               cpu_max_ext_state_size);
   
           /*
            * Create a new fresh stack for the new thread.
            */
           bcopy(td0->td_frame, td->td_frame, sizeof(struct trapframe));
   
           /* If the current thread has the trap bit set (i.e. a debugger had
            * single stepped the process to the system call), we need to clear
            * the trap flag from the new frame. Otherwise, the new thread will
            * receive a (likely unexpected) SIGTRAP when it executes the first
            * instruction after returning to userland.
            */
           td->td_frame->tf_eflags &= ~PSL_T;
   
           /*
            * Set registers for trampoline to user mode.  Leave space for the
            * return address on stack.  These are the kernel mode register values.
            */
           pcb2->pcb_edi = 0;
           pcb2->pcb_esi = (int)fork_return;                   /* trampoline arg */
           pcb2->pcb_ebp = 0;
           pcb2->pcb_esp = (int)td->td_frame - sizeof(void *); /* trampoline arg */
           pcb2->pcb_ebx = (int)td;                            /* trampoline arg */
           pcb2->pcb_eip = (int)fork_trampoline + setidt_disp;
           pcb2->pcb_gs = rgs();
           /*
            * If we didn't copy the pcb, we'd need to do the following registers:
            * pcb2->pcb_cr3:       cloned above.
            * pcb2->pcb_dr*:       cloned above.
            * pcb2->pcb_savefpu:   cloned above.
            * pcb2->pcb_flags:     cloned above.
            * pcb2->pcb_onfault:   cloned above (always NULL here?).
            * pcb2->pcb_gs:        cloned above.
            * pcb2->pcb_ext:       cleared below.
            */
           pcb2->pcb_ext = NULL;
   
           /* Setup to release spin count in fork_exit(). */
           td->td_md.md_spinlock_count = 1;
           td->td_md.md_saved_flags = PSL_KERNEL | PSL_I;
   }
   
   /*
    * Set that machine state for performing an upcall that starts
    * the entry function with the given argument.
    */
   void
   cpu_set_upcall(struct thread *td, void (*entry)(void *), void *arg,
       stack_t *stack)
   {
   
           /* 
            * Do any extra cleaning that needs to be done.
            * The thread may have optional components
            * that are not present in a fresh thread.
            * This may be a recycled thread so make it look
            * as though it's newly allocated.
            */
           cpu_thread_clean(td);
   
           /*
            * Set the trap frame to point at the beginning of the entry
            * function.
            */
           td->td_frame->tf_ebp = 0; 
           td->td_frame->tf_esp =
               (((int)stack->ss_sp + stack->ss_size - 4) & ~0x0f) - 4;
           td->td_frame->tf_eip = (int)entry;
   
           /* Return address sentinel value to stop stack unwinding. */
           suword((void *)td->td_frame->tf_esp, 0);
   
           /* Pass the argument to the entry point. */
           suword((void *)(td->td_frame->tf_esp + sizeof(void *)),
               (int)arg);
   }
   
   int
   cpu_set_user_tls(struct thread *td, void *tls_base)
   {
           struct segment_descriptor sd;
           uint32_t base;
   
           /*
            * Construct a descriptor and store it in the pcb for
            * the next context switch.  Also store it in the gdt
            * so that the load of tf_fs into %fs will activate it
            * at return to userland.
            */
           base = (uint32_t)tls_base;
           sd.sd_lobase = base & 0xffffff;
           sd.sd_hibase = (base >> 24) & 0xff;
           sd.sd_lolimit = 0xffff; /* 4GB limit, wraps around */
           sd.sd_hilimit = 0xf;
           sd.sd_type  = SDT_MEMRWA;
           sd.sd_dpl   = SEL_UPL;
           sd.sd_p     = 1;
           sd.sd_xx    = 0;
           sd.sd_def32 = 1;
           sd.sd_gran  = 1;
           critical_enter();
           /* set %gs */
           td->td_pcb->pcb_gsd = sd;
           if (td == curthread) {
                   PCPU_GET(fsgs_gdt)[1] = sd;
                   load_gs(GSEL(GUGS_SEL, SEL_UPL));
           }
           critical_exit();
           return (0);
   }
   
   /*
    * Convert kernel VA to physical address
    */
   vm_paddr_t
   kvtop(void *addr)
   {
           vm_paddr_t pa;
   
           pa = pmap_kextract((vm_offset_t)addr);
           if (pa == 0)
                   panic("kvtop: zero page frame");
           return (pa);
   }
   
   /*
    * Get an sf_buf from the freelist.  May block if none are available.
    */
   void
   sf_buf_map(struct sf_buf *sf, int flags)
   {
           pt_entry_t opte, *ptep;
   
           /*
            * Update the sf_buf's virtual-to-physical mapping, flushing the
            * virtual address from the TLB.  Since the reference count for 
            * the sf_buf's old mapping was zero, that mapping is not 
            * currently in use.  Consequently, there is no need to exchange 
            * the old and new PTEs atomically, even under PAE.
            */
           ptep = vtopte(sf->kva);
           opte = *ptep;
           *ptep = VM_PAGE_TO_PHYS(sf->m) | PG_RW | PG_V |
               pmap_cache_bits(kernel_pmap, sf->m->md.pat_mode, 0);
   
           /*
            * Avoid unnecessary TLB invalidations: If the sf_buf's old
            * virtual-to-physical mapping was not used, then any processor
            * that has invalidated the sf_buf's virtual address from its TLB
            * since the last used mapping need not invalidate again.
            */
   #ifdef SMP
           if ((opte & (PG_V | PG_A)) ==  (PG_V | PG_A))
                   CPU_ZERO(&sf->cpumask);
   
           sf_buf_shootdown(sf, flags);
   #else
           if ((opte & (PG_V | PG_A)) ==  (PG_V | PG_A))
                   pmap_invalidate_page(kernel_pmap, sf->kva);
   #endif
   }
   
   #ifdef SMP
   void
   sf_buf_shootdown(struct sf_buf *sf, int flags)
   {
           cpuset_t other_cpus;
           u_int cpuid;
   
           sched_pin();
           cpuid = PCPU_GET(cpuid);
           if (!CPU_ISSET(cpuid, &sf->cpumask)) {
                   CPU_SET(cpuid, &sf->cpumask);
                   invlpg(sf->kva);
           }
           if ((flags & SFB_CPUPRIVATE) == 0) {
                   other_cpus = all_cpus;
                   CPU_CLR(cpuid, &other_cpus);
                   CPU_NAND(&other_cpus, &sf->cpumask);
                   if (!CPU_EMPTY(&other_cpus)) {
                           CPU_OR(&sf->cpumask, &other_cpus);
                           smp_masked_invlpg(other_cpus, sf->kva, kernel_pmap);
                   }
           }
           sched_unpin();
   }
   #endif
   
   /*
    * MD part of sf_buf_free().
    */
   int
   sf_buf_unmap(struct sf_buf *sf)
   {
   
           return (0);
   }
   
   static void
   sf_buf_invalidate(struct sf_buf *sf)
   {
           vm_page_t m = sf->m;
   
           /*
            * Use pmap_qenter to update the pte for
            * existing mapping, in particular, the PAT
            * settings are recalculated.
            */
           pmap_qenter(sf->kva, &m, 1);
           pmap_invalidate_cache_range(sf->kva, sf->kva + PAGE_SIZE);
   }
   
   /*
    * Invalidate the cache lines that may belong to the page, if
    * (possibly old) mapping of the page by sf buffer exists.  Returns
    * TRUE when mapping was found and cache invalidated.
    */
   boolean_t
   sf_buf_invalidate_cache(vm_page_t m)
   {
   
           return (sf_buf_process_page(m, sf_buf_invalidate));
   }
   
   /*
    * Software interrupt handler for queued VM system processing.
    */   
   void  
   swi_vm(void *dummy) 
   {     
           if (busdma_swi_pending != 0)
                   busdma_swi();
   }
   
   /*
    * Tell whether this address is in some physical memory region.
    * Currently used by the kernel coredump code in order to avoid
    * dumping the ``ISA memory hole'' which could cause indefinite hangs,
    * or other unpredictable behaviour.
    */
   
   int
   is_physical_memory(vm_paddr_t addr)
   {
   
   #ifdef DEV_ISA
           /* The ISA ``memory hole''. */
           if (addr >= 0xa0000 && addr < 0x100000)
                   return 0;
   #endif
   
           /*
            * stuff other tests for known memory-mapped devices (PCI?)
            * here
            */
   
           return 1;
   }

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