diff options
| -rw-r--r-- | block/io.c | 62 | ||||
| -rw-r--r-- | include/block/block-io.h | 2 |
2 files changed, 64 insertions, 0 deletions
diff --git a/block/io.c b/block/io.c index 50dc0e193f..4fd7768f9c 100644 --- a/block/io.c +++ b/block/io.c @@ -38,10 +38,14 @@ #include "qemu/error-report.h" #include "qemu/main-loop.h" #include "system/replay.h" +#include "qemu/units.h" /* Maximum bounce buffer for copy-on-read and write zeroes, in bytes */ #define MAX_BOUNCE_BUFFER (32768 << BDRV_SECTOR_BITS) +/* Maximum read size for checking if data reads as zero, in bytes */ +#define MAX_ZERO_CHECK_BUFFER (128 * KiB) + static void coroutine_fn GRAPH_RDLOCK bdrv_parent_cb_resize(BlockDriverState *bs); @@ -2778,6 +2782,64 @@ int coroutine_fn bdrv_co_is_zero_fast(BlockDriverState *bs, int64_t offset, return 1; } +/* + * Check @bs (and its backing chain) to see if the entire image is known + * to read as zeroes. + * Return 1 if that is the case, 0 otherwise and -errno on error. + * This test is meant to be fast rather than accurate so returning 0 + * does not guarantee non-zero data; however, a return of 1 is reliable, + * and this function can report 1 in more cases than bdrv_co_is_zero_fast. + */ +int coroutine_fn bdrv_co_is_all_zeroes(BlockDriverState *bs) +{ + int ret; + int64_t pnum, bytes; + char *buf; + QEMUIOVector local_qiov; + IO_CODE(); + + bytes = bdrv_co_getlength(bs); + if (bytes < 0) { + return bytes; + } + + /* First probe - see if the entire image reads as zero */ + ret = bdrv_co_common_block_status_above(bs, NULL, false, BDRV_WANT_ZERO, + 0, bytes, &pnum, NULL, NULL, + NULL); + if (ret < 0) { + return ret; + } + if (ret & BDRV_BLOCK_ZERO) { + return bdrv_co_is_zero_fast(bs, pnum, bytes - pnum); + } + + /* + * Because of the way 'blockdev-create' works, raw files tend to + * be created with a non-sparse region at the front to make + * alignment probing easier. If the block starts with only a + * small allocated region, it is still worth the effort to see if + * the rest of the image is still sparse, coupled with manually + * reading the first region to see if it reads zero after all. + */ + if (pnum > MAX_ZERO_CHECK_BUFFER) { + return 0; + } + ret = bdrv_co_is_zero_fast(bs, pnum, bytes - pnum); + if (ret <= 0) { + return ret; + } + /* Only the head of the image is unknown, and it's small. Read it. */ + buf = qemu_blockalign(bs, pnum); + qemu_iovec_init_buf(&local_qiov, buf, pnum); + ret = bdrv_driver_preadv(bs, 0, pnum, &local_qiov, 0, 0); + if (ret >= 0) { + ret = buffer_is_zero(buf, pnum); + } + qemu_vfree(buf); + return ret; +} + int coroutine_fn bdrv_co_is_allocated(BlockDriverState *bs, int64_t offset, int64_t bytes, int64_t *pnum) { diff --git a/include/block/block-io.h b/include/block/block-io.h index b49e0537dd..b99cc98d26 100644 --- a/include/block/block-io.h +++ b/include/block/block-io.h @@ -161,6 +161,8 @@ bdrv_is_allocated_above(BlockDriverState *bs, BlockDriverState *base, int coroutine_fn GRAPH_RDLOCK bdrv_co_is_zero_fast(BlockDriverState *bs, int64_t offset, int64_t bytes); +int coroutine_fn GRAPH_RDLOCK +bdrv_co_is_all_zeroes(BlockDriverState *bs); int GRAPH_RDLOCK bdrv_apply_auto_read_only(BlockDriverState *bs, const char *errmsg, |