LXR qemu/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/osdep.h"
  13#include <qemu/mmap-alloc.h>
  14#include <sys/mman.h>
  15
  16#define HUGETLBFS_MAGIC       0x958458f6
  17
  18#ifdef CONFIG_LINUX
  19#include <sys/vfs.h>
  20#endif
  21
  22size_t qemu_fd_getpagesize(int fd)
  23{
  24#ifdef CONFIG_LINUX
  25    struct statfs fs;
  26    int ret;
  27
  28    if (fd != -1) {
  29        do {
  30            ret = fstatfs(fd, &fs);
  31        } while (ret != 0 && errno == EINTR);
  32
  33        if (ret == 0 && fs.f_type == HUGETLBFS_MAGIC) {
  34            return fs.f_bsize;
  35        }
  36    }
  37#endif
  38
  39    return getpagesize();
  40}
  41
  42void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared)
  43{
  44    /*
  45     * Note: this always allocates at least one extra page of virtual address
  46     * space, even if size is already aligned.
  47     */
  48    size_t total = size + align;
  49#if defined(__powerpc64__) && defined(__linux__)
  50    /* On ppc64 mappings in the same segment (aka slice) must share the same
  51     * page size. Since we will be re-allocating part of this segment
  52     * from the supplied fd, we should make sure to use the same page size, to
  53     * this end we mmap the supplied fd.  In this case, set MAP_NORESERVE to
  54     * avoid allocating backing store memory.
  55     * We do this unless we are using the system page size, in which case
  56     * anonymous memory is OK.
  57     */
  58    int anonfd = fd == -1 || qemu_fd_getpagesize(fd) == getpagesize() ? -1 : fd;
  59    int flags = anonfd == -1 ? MAP_ANONYMOUS : MAP_NORESERVE;
  60    void *ptr = mmap(0, total, PROT_NONE, flags | MAP_PRIVATE, anonfd, 0);
  61#else
  62    void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
  63#endif
  64    size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
  65    void *ptr1;
  66
  67    if (ptr == MAP_FAILED) {
  68        return MAP_FAILED;
  69    }
  70
  71    /* Make sure align is a power of 2 */
  72    assert(!(align & (align - 1)));
  73    /* Always align to host page size */
  74    assert(align >= getpagesize());
  75
  76    ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE,
  77                MAP_FIXED |
  78                (fd == -1 ? MAP_ANONYMOUS : 0) |
  79                (shared ? MAP_SHARED : MAP_PRIVATE),
  80                fd, 0);
  81    if (ptr1 == MAP_FAILED) {
  82        munmap(ptr, total);
  83        return MAP_FAILED;
  84    }
  85
  86    ptr += offset;
  87    total -= offset;
  88
  89    if (offset > 0) {
  90        munmap(ptr - offset, offset);
  91    }
  92
  93    /*
  94     * Leave a single PROT_NONE page allocated after the RAM block, to serve as
  95     * a guard page guarding against potential buffer overflows.
  96     */
  97    if (total > size + getpagesize()) {
  98        munmap(ptr + size + getpagesize(), total - size - getpagesize());
  99    }
 100
 101    return ptr;
 102}
 103
 104void qemu_ram_munmap(void *ptr, size_t size)
 105{
 106    if (ptr) {
 107        /* Unmap both the RAM block and the guard page */
 108        munmap(ptr, size + getpagesize());
 109    }
 110}
 111