[Qemu-devel] [PATCH] block: Avoid unecessary drv->bdrv_getlength() calls

[Kevin Wolf <kwolf@redhat.com>]

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; 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>
---
 block.c                   | 7 ++++---
 block/raw-posix.c         | 9 ++++++---
 block/raw_bsd.c           | 1 +
 include/block/block_int.h | 3 +++
 4 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/block.c b/block.c
index 366999b..da88be0 100644
--- a/block.c
+++ b/block.c
@@ -2868,9 +2868,10 @@ 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;
diff --git a/block/raw-posix.c b/block/raw-posix.c
index 6f03fbf..f6d48bb 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -1715,7 +1715,8 @@ static BlockDriver bdrv_host_floppy = {
     .bdrv_aio_flush	= raw_aio_flush,
 
     .bdrv_truncate      = raw_truncate,
-    .bdrv_getlength	= raw_getlength,
+    .bdrv_getlength      = raw_getlength,
+    .has_variable_length = true,
     .bdrv_get_allocated_file_size
                         = raw_get_allocated_file_size,
 
@@ -1824,7 +1825,8 @@ static BlockDriver bdrv_host_cdrom = {
     .bdrv_aio_flush	= raw_aio_flush,
 
     .bdrv_truncate      = raw_truncate,
-    .bdrv_getlength     = raw_getlength,
+    .bdrv_getlength      = raw_getlength,
+    .has_variable_length = true,
     .bdrv_get_allocated_file_size
                         = raw_get_allocated_file_size,
 
@@ -1951,7 +1953,8 @@ static BlockDriver bdrv_host_cdrom = {
     .bdrv_aio_flush	= raw_aio_flush,
 
     .bdrv_truncate      = raw_truncate,
-    .bdrv_getlength     = raw_getlength,
+    .bdrv_getlength      = raw_getlength,
+    .has_variable_length = true,
     .bdrv_get_allocated_file_size
                         = raw_get_allocated_file_size,
 
diff --git a/block/raw_bsd.c b/block/raw_bsd.c
index 0078c1b..2265dcc 100644
--- a/block/raw_bsd.c
+++ b/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,
diff --git a/include/block/block_int.h b/include/block/block_int.h
index a48731d..1666066 100644
--- a/include/block/block_int.h
+++ b/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);
 
-- 
1.8.1.4