diff options
| -rw-r--r-- | hw/block/block.c | 36 |
1 files changed, 35 insertions, 1 deletions
diff --git a/hw/block/block.c b/hw/block/block.c index ddcef71f80..af0710e477 100644 --- a/hw/block/block.c +++ b/hw/block/block.c @@ -16,6 +16,40 @@ #include "qapi/qapi-types-block.h" /* + * Read the non-zeroes parts of @blk into @buf + * Reading all of the @blk is expensive if the zeroes parts of @blk + * is large enough. Therefore check the block status and only write + * the non-zeroes block into @buf. + * + * Return 0 on success, non-zero on error. + */ +static int blk_pread_nonzeroes(BlockBackend *blk, hwaddr size, void *buf) +{ + int ret; + int64_t bytes, offset = 0; + BlockDriverState *bs = blk_bs(blk); + + for (;;) { + bytes = MIN(size - offset, BDRV_REQUEST_MAX_SECTORS); + if (bytes <= 0) { + return 0; + } + ret = bdrv_block_status(bs, offset, bytes, &bytes, NULL, NULL); + if (ret < 0) { + return ret; + } + if (!(ret & BDRV_BLOCK_ZERO)) { + ret = bdrv_pread(bs->file, offset, bytes, + (uint8_t *) buf + offset, 0); + if (ret < 0) { + return ret; + } + } + offset += bytes; + } +} + +/* * Read the entire contents of @blk into @buf. * @blk's contents must be @size bytes, and @size must be at most * BDRV_REQUEST_MAX_BYTES. @@ -54,7 +88,7 @@ bool blk_check_size_and_read_all(BlockBackend *blk, void *buf, hwaddr size, * block device and read only on demand. */ assert(size <= BDRV_REQUEST_MAX_BYTES); - ret = blk_pread(blk, 0, size, buf, 0); + ret = blk_pread_nonzeroes(blk, size, buf); if (ret < 0) { error_setg_errno(errp, -ret, "can't read block backend"); return false; |