util/mmap-alloc.c

   1 /*
   2  * Support for RAM backed by mmaped host memory.
   3  *
   4  * Copyright (c) 2015 Red Hat, Inc.
   5  *
   6  * Authors:
   7  *  Michael S. Tsirkin <mst@redhat.com>
   8  *
   9  * This work is licensed under the terms of the GNU GPL, version 2 or
  10  * later.  See the COPYING file in the top-level directory.
  11  */
  12 #include <qemu/mmap-alloc.h>
  13 #include <sys/types.h>
  14 #include <sys/mman.h>
  15 #include <assert.h>
  16
  17 void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared)
  18 {
  19     /*
  20      * Note: this always allocates at least one extra page of virtual address
  21      * space, even if size is already aligned.
  22      */
  23     size_t total = size + align;
  24     void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
  25     size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
  26     void *ptr1;
  27
  28     if (ptr == MAP_FAILED) {
  29         return MAP_FAILED;
  30     }
  31
  32     /* Make sure align is a power of 2 */
  33     assert(!(align & (align - 1)));
  34     /* Always align to host page size */
  35     assert(align >= getpagesize());
  36
  37     ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE,
  38                 MAP_FIXED |
  39                 (fd == -1 ? MAP_ANONYMOUS : 0) |
  40                 (shared ? MAP_SHARED : MAP_PRIVATE),
  41                 fd, 0);
  42     if (ptr1 == MAP_FAILED) {
  43         munmap(ptr, total);
  44         return MAP_FAILED;
  45     }
  46
  47     ptr += offset;
  48     total -= offset;
  49
  50     if (offset > 0) {
  51         munmap(ptr - offset, offset);
  52     }
  53
  54     /*
  55      * Leave a single PROT_NONE page allocated after the RAM block, to serve as
  56      * a guard page guarding against potential buffer overflows.
  57      */
  58     if (total > size + getpagesize()) {
  59         munmap(ptr + size + getpagesize(), total - size - getpagesize());
  60     }
  61
  62     return ptr;
  63 }
  64
  65 void qemu_ram_munmap(void *ptr, size_t size)
  66 {
  67     if (ptr) {
  68         /* Unmap both the RAM block and the guard page */
  69         munmap(ptr, size + getpagesize());
  70     }
  71 }