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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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