From f0d6b82f196bffe17102627696ae3158e476a110 Mon Sep 17 00:00:00 2001 Message-Id: In-Reply-To: <276ddced7c9181cce17d0ff9eb080f99dcfe0ac3.1376492227.git.minovotn@redhat.com> References: <276ddced7c9181cce17d0ff9eb080f99dcfe0ac3.1376492227.git.minovotn@redhat.com> From: Asias He Date: Wed, 14 Aug 2013 10:24:16 +0200 Subject: [PATCH 15/22] block: Produce zeros when protocols reading beyond end of file RH-Author: Asias He Message-id: <1376475863-27929-11-git-send-email-asias@redhat.com> Patchwork-id: 53397 O-Subject: [RHEL6.5 qemu-kvm PATCH v4 10/17] block: Produce zeros when protocols reading beyond end of file Bugzilla: 848070 RH-Acked-by: Kevin Wolf RH-Acked-by: Stefan Hajnoczi RH-Acked-by: Jeffrey Cody From: MORITA Kazutaka While Asias is debugging an issue creating qcow2 images on top of non-file protocols. It boils down to this example using NBD: $ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512' Notice the open -g option to set bs->growable. This means you can read/write beyond end of file. Reading beyond end of file is supposed to produce zeroes. We rely on this behavior in qcow2_create2() during qcow2 image creation. We create a new file and then write the qcow2 header structure using bdrv_pwrite(). Since QCowHeader is not a multiple of sector size, block.c first uses bdrv_read() on the empty file to fetch the first sector (should be all zeroes). Here is the output from the qemu-io NBD example above: $ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512' 00000000: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................ 00000010: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................ 00000020: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................ ... We are not zeroing the buffer! As a result qcow2 image creation on top of protocols is not guaranteed to work even when file creation is supported by the protocol. Signed-off-by: MORITA Kazutaka Signed-off-by: Asias He Signed-off-by: Stefan Hajnoczi (cherry picked from commit 4146b46c42e0989cb5842e04d88ab6ccb1713a48) Use qemu_iovec_memset_skip(qiov, 0, bytes, offset * BDRV_SECTOR_SIZE) instead of qemu_iovec_memset(qiov, offset * BDRV_SECTOR_SIZE, 0, bytes) in RHEL6. --- block.c | 30 +++++++++++++++++++++++++++++- 1 file changed, 29 insertions(+), 1 deletion(-) Signed-off-by: Michal Novotny --- block.c | 30 +++++++++++++++++++++++++++++- 1 file changed, 29 insertions(+), 1 deletion(-) diff --git a/block.c b/block.c index 2aa6e34..5c5e9f7 100644 --- a/block.c +++ b/block.c @@ -2163,7 +2163,35 @@ static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs, } } - ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov); + if (!bs->growable) { + ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov); + } else { + /* Read zeros after EOF of growable BDSes */ + int64_t len, total_sectors, max_nb_sectors; + + len = bdrv_getlength(bs); + if (len < 0) { + ret = len; + goto out; + } + + total_sectors = len >> BDRV_SECTOR_BITS; + max_nb_sectors = MAX(0, total_sectors - sector_num); + if (max_nb_sectors > 0) { + ret = drv->bdrv_co_readv(bs, sector_num, + MIN(nb_sectors, max_nb_sectors), qiov); + } else { + ret = 0; + } + + /* Reading beyond end of file is supposed to produce zeroes */ + if (ret == 0 && total_sectors < sector_num + nb_sectors) { + uint64_t offset = MAX(0, total_sectors - sector_num); + uint64_t bytes = (sector_num + nb_sectors - offset) * + BDRV_SECTOR_SIZE; + qemu_iovec_memset_skip(qiov, 0, bytes, offset * BDRV_SECTOR_SIZE); + } + } out: tracked_request_end(&req); -- 1.7.11.7