/* * 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 MAP_FAILED; } /* 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 MAP_FAILED; } 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()); } }