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>

diff --git a/block.c b/block.c
index 61795fea46401fe7677d82556ed54204e9fafb56..58efb5b4e474b30c87692557c6bd63e526ad693a 100644 (file)
--- a/block.c
+++ b/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;

diff --git a/block/raw-posix.c b/block/raw-posix.c
index 6f03fbf7931f1038893b48ec8490178055865485..f6d48bbdb2d685074d547e6a28493c7f4519f9a2 100644 (file)
--- 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-win32.c b/block/raw-win32.c
index 676b5701db6700c6c5a588783bc9a8c7a58a7409..2741e4de10b9989ca916bfe90357680c2f4fbf54 100644 (file)
--- a/block/raw-win32.c
+++ b/block/raw-win32.c
@@ -616,7 +616,9 @@ static BlockDriver bdrv_host_device = {
     .bdrv_aio_writev    = raw_aio_writev,
     .bdrv_aio_flush     = raw_aio_flush,
 
-    .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 0078c1baebf2f17ad804b20fabfbb7d177fb17f2..2265dcc03faf033d3fbe7d9609ba7598db803c06 100644 (file)
--- 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 a48731d539912e1dc4af67af103a98f259255684..166606615c75cb7b2868499cb46f11502be3de76 100644 (file)
--- 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);