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From 12e88a4a74931578a5582b5caeca135fe85fb01b Mon Sep 17 00:00:00 2001
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From: Asias He <asias@redhat.com>
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Date: Thu, 12 Sep 2013 07:39:34 +0200
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Subject: [PATCH 14/29] block: Produce zeros when protocols reading beyond end of file
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RH-Author: Asias He <asias@redhat.com>
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Message-id: <1378971575-22416-4-git-send-email-asias@redhat.com>
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Patchwork-id: 54325
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O-Subject: [RHEL7.0 qemu-kvm PATCH 3/4] block: Produce zeros when protocols reading beyond end of file
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Bugzilla: 1007226
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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RH-Acked-by: Fam Zheng <famz@redhat.com>
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From: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
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BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1005052
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Brew: https://brewweb.devel.redhat.com/taskinfo?taskID=6275752
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While Asias is debugging an issue creating qcow2 images on top of
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non-file protocols. It boils down to this example using NBD:
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$ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'
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Notice the open -g option to set bs->growable. This means you can
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read/write beyond end of file. Reading beyond end of file is supposed
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to produce zeroes.
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We rely on this behavior in qcow2_create2() during qcow2 image
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creation. We create a new file and then write the qcow2 header
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structure using bdrv_pwrite(). Since QCowHeader is not a multiple of
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sector size, block.c first uses bdrv_read() on the empty file to fetch
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the first sector (should be all zeroes).
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Here is the output from the qemu-io NBD example above:
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$ qemu-io -c 'open -g nbd+unix:///?socket=/tmp/nbd.sock' -c 'read -v 0 512'
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00000000: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
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00000010: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
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00000020: ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ab ................
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...
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We are not zeroing the buffer! As a result qcow2 image creation on top
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of protocols is not guaranteed to work even when file creation is
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supported by the protocol.
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[Adapted this patch to use bs->zero_beyond_eof.
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-- Stefan]
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Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
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Signed-off-by: Asias He <asias@redhat.com>
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Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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(cherry picked from commit 893a8f6220368a9ebff9a74bd48359928545cf6a)
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---
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block.c | 30 +++++++++++++++++++++++++++++-
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1 file changed, 29 insertions(+), 1 deletion(-)
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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block.c | 30 +++++++++++++++++++++++++++++-
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1 files changed, 29 insertions(+), 1 deletions(-)
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diff --git a/block.c b/block.c
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index dbcad0e..a3bc800 100644
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--- a/block.c
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+++ b/block.c
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@@ -2555,7 +2555,35 @@ static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs,
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}
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}
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- ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
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+ if (!(bs->zero_beyond_eof && bs->growable)) {
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+ ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
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+ } else {
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+ /* Read zeros after EOF of growable BDSes */
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+ int64_t len, total_sectors, max_nb_sectors;
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+
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+ len = bdrv_getlength(bs);
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+ if (len < 0) {
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+ ret = len;
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+ goto out;
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+ }
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+
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+ total_sectors = len >> BDRV_SECTOR_BITS;
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+ max_nb_sectors = MAX(0, total_sectors - sector_num);
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+ if (max_nb_sectors > 0) {
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+ ret = drv->bdrv_co_readv(bs, sector_num,
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+ MIN(nb_sectors, max_nb_sectors), qiov);
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+ } else {
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+ ret = 0;
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+ }
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+
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+ /* Reading beyond end of file is supposed to produce zeroes */
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+ if (ret == 0 && total_sectors < sector_num + nb_sectors) {
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+ uint64_t offset = MAX(0, total_sectors - sector_num);
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+ uint64_t bytes = (sector_num + nb_sectors - offset) *
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+ BDRV_SECTOR_SIZE;
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+ qemu_iovec_memset(qiov, offset * BDRV_SECTOR_SIZE, 0, bytes);
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+ }
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+ }
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out:
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tracked_request_end(&req;;
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--
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1.7.1
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