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hostmem-file: add offset option

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Add an option for hostmem-file to start the memory object at an offset
into the target file. This is useful if multiple memory objects reside
inside the same target file, such as a device node.

In particular, it's useful to map guest memory directly into /dev/mem
for experimentation.

To make this work consistently, also fix up all places in QEMU that
expect fd offsets to be 0.

Signed-off-by: Alexander Graf <graf@amazon.com>
Message-Id: <20230403221421.60877-1-graf@amazon.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
This commit is contained in:
Alexander Graf 2023-04-03 22:14:21 +00:00 • committed by David Hildenbrand
parent 886c0453cb
commit 4b870dc4d0
9 changed files with 69 additions and 9 deletions
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17
softmmu/physmem.c
View file

@ -1369,6 +1369,11 @@ static void *file_ram_alloc(RAMBlock *block,
error_setg(errp, "alignment 0x%" PRIx64
" must be a power of two", block->mr->align);
return NULL;
} else if (offset % block->page_size) {
error_setg(errp, "offset 0x%" PRIx64
" must be multiples of page size 0x%zx",
offset, block->page_size);
return NULL;
}
block->mr->align = MAX(block->page_size, block->mr->align);
#if defined(__s390x__)
@ -1400,7 +1405,7 @@ static void *file_ram_alloc(RAMBlock *block,
* those labels. Therefore, extending the non-empty backend file
* is disabled as well.
*/
if (truncate && ftruncate(fd, memory)) {
if (truncate && ftruncate(fd, offset + memory)) {
perror("ftruncate");
}
@ -1416,6 +1421,7 @@ static void *file_ram_alloc(RAMBlock *block,
}
block->fd = fd;
block->fd_offset = offset;
return area;
}
#endif
@ -1889,7 +1895,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
size = HOST_PAGE_ALIGN(size);
file_size = get_file_size(fd);
if (file_size > 0 && file_size < size) {
if (file_size > offset && file_size < (offset + size)) {
error_setg(errp, "backing store size 0x%" PRIx64
" does not match 'size' option 0x" RAM_ADDR_FMT,
file_size, size);
@ -1929,7 +1935,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
uint32_t ram_flags, const char *mem_path,
bool readonly, Error **errp)
off_t offset, bool readonly, Error **errp)
{
int fd;
bool created;
@ -1941,7 +1947,8 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
return NULL;
}
block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, 0, readonly, errp);
block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, offset, readonly,
errp);
if (!block) {
if (created) {
unlink(mem_path);
@ -2075,7 +2082,7 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
flags |= block->flags & RAM_NORESERVE ? MAP_NORESERVE : 0;
if (block->fd >= 0) {
area = mmap(vaddr, length, PROT_READ | PROT_WRITE,
flags, block->fd, offset);
flags, block->fd, offset + block->fd_offset);
} else {
flags |= MAP_ANONYMOUS;
area = mmap(vaddr, length, PROT_READ | PROT_WRITE,