From b94a2610573cd9314f244207c8b04cb75e42d7f8 Mon Sep 17 00:00:00 2001 From: Kevin Wolf Date: Tue, 29 Oct 2013 12:18:58 +0100 Subject: [PATCH] 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 Reviewed-by: Paolo Bonzini --- block.c | 7 ++++--- block/raw-posix.c | 9 ++++++--- block/raw-win32.c | 4 +++- block/raw_bsd.c | 1 + include/block/block_int.h | 3 +++ 5 files changed, 17 insertions(+), 7 deletions(-) diff --git a/block.c b/block.c index 61795fea46..58efb5b4e4 100644 --- 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 6f03fbf793..f6d48bbdb2 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-win32.c b/block/raw-win32.c index 676b5701db..2741e4de10 100644 --- 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 0078c1baeb..2265dcc03f 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 a48731d539..166606615c 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); -- 2.39.2