block: Avoid unecessary drv->bdrv_getlength() calls

The block layer generally keeps the size of an image cached in
bs->total_sectors so that it doesn't have to perform expensive
operations to get the size whenever it needs it.

This doesn't work however when using a backend that can change its size
without qemu being aware of it, i.e. passthrough of removable media like
CD-ROMs or floppy disks. For this reason, the caching is disabled when a
removable device is used.

It is obvious that checking whether the _guest_ device has removable
media isn't the right thing to do when we want to know whether the size
of the host backend can change. To make things worse, non-top-level
BlockDriverStates never have any device attached, which makes qemu
assume they are removable, so drv->bdrv_getlength() is always called on
the protocol layer. In the case of raw-posix, this causes unnecessary
lseek() system calls, which turned out to be rather expensive.

This patch completely changes the logic and disables bs->total_sectors
caching only for certain block driver types, for which a size change is
expected: host_cdrom and host_floppy on POSIX, host_device on win32; also
the raw format in case it sits on top of one of these protocols, but in
the common case the nested bdrv_getlength() call on the protocol driver
will use the cache again and avoid an expensive drv->bdrv_getlength()
call.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Kevin Wolf 2013-10-29 12:18:58 +01:00
parent 87a5debd31
commit b94a261057
5 changed files with 17 additions and 7 deletions

View File

@ -2869,9 +2869,10 @@ int64_t bdrv_getlength(BlockDriverState *bs)
if (!drv)
return -ENOMEDIUM;
if (bdrv_dev_has_removable_media(bs)) {
if (drv->bdrv_getlength) {
return drv->bdrv_getlength(bs);
if (drv->has_variable_length) {
int ret = refresh_total_sectors(bs, bs->total_sectors);
if (ret < 0) {
return ret;
}
}
return bs->total_sectors * BDRV_SECTOR_SIZE;

View File

@ -1716,6 +1716,7 @@ static BlockDriver bdrv_host_floppy = {
.bdrv_truncate = raw_truncate,
.bdrv_getlength = raw_getlength,
.has_variable_length = true,
.bdrv_get_allocated_file_size
= raw_get_allocated_file_size,
@ -1825,6 +1826,7 @@ static BlockDriver bdrv_host_cdrom = {
.bdrv_truncate = raw_truncate,
.bdrv_getlength = raw_getlength,
.has_variable_length = true,
.bdrv_get_allocated_file_size
= raw_get_allocated_file_size,
@ -1952,6 +1954,7 @@ static BlockDriver bdrv_host_cdrom = {
.bdrv_truncate = raw_truncate,
.bdrv_getlength = raw_getlength,
.has_variable_length = true,
.bdrv_get_allocated_file_size
= raw_get_allocated_file_size,

View File

@ -617,6 +617,8 @@ static BlockDriver bdrv_host_device = {
.bdrv_aio_flush = raw_aio_flush,
.bdrv_getlength = raw_getlength,
.has_variable_length = true,
.bdrv_get_allocated_file_size
= raw_get_allocated_file_size,
};

View File

@ -178,6 +178,7 @@ static BlockDriver bdrv_raw = {
.bdrv_co_get_block_status = &raw_co_get_block_status,
.bdrv_truncate = &raw_truncate,
.bdrv_getlength = &raw_getlength,
.has_variable_length = true,
.bdrv_get_info = &raw_get_info,
.bdrv_is_inserted = &raw_is_inserted,
.bdrv_media_changed = &raw_media_changed,

View File

@ -156,8 +156,11 @@ struct BlockDriver {
const char *protocol_name;
int (*bdrv_truncate)(BlockDriverState *bs, int64_t offset);
int64_t (*bdrv_getlength)(BlockDriverState *bs);
bool has_variable_length;
int64_t (*bdrv_get_allocated_file_size)(BlockDriverState *bs);
int (*bdrv_write_compressed)(BlockDriverState *bs, int64_t sector_num,
const uint8_t *buf, int nb_sectors);