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/*
* Support for RAM backed by mmaped host memory.
*
* Copyright (c) 2015 Red Hat, Inc.
*
* Authors:
* Michael S. Tsirkin <mst@redhat.com>
*
* This work is licensed under the terms of the GNU GPL, version 2 or
* later. See the COPYING file in the top-level directory.
*/
#include <qemu/mmap-alloc.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <assert.h>
void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared)
{
/*
* Note: this always allocates at least one extra page of virtual address
* space, even if size is already aligned.
*/
size_t total = size + align;
void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
void *ptr1;
if (ptr == MAP_FAILED) {
return NULL;
}
/* Make sure align is a power of 2 */
assert(!(align & (align - 1)));
/* Always align to host page size */
assert(align >= getpagesize());
ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE,
MAP_FIXED |
(fd == -1 ? MAP_ANONYMOUS : 0) |
(shared ? MAP_SHARED : MAP_PRIVATE),
fd, 0);
if (ptr1 == MAP_FAILED) {
munmap(ptr, total);
return NULL;
}
ptr += offset;
total -= offset;
if (offset > 0) {
munmap(ptr - offset, offset);
}
/*
* Leave a single PROT_NONE page allocated after the RAM block, to serve as
* a guard page guarding against potential buffer overflows.
*/
if (total > size + getpagesize()) {
munmap(ptr + size + getpagesize(), total - size - getpagesize());
}
return ptr;
}
void qemu_ram_munmap(void *ptr, size_t size)
{
if (ptr) {
/* Unmap both the RAM block and the guard page */
munmap(ptr, size + getpagesize());
}
}
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