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>

block.c

@@ -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;

block/raw-posix.c

@@ -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,
 

block/raw-win32.c

@@ -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,
 };

block/raw_bsd.c

@@ -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,

include/block/block_int.h

@@ -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);