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Description of problem:
Offline migration should not fail with error "Migration may lead to data corruption if disks use cache != none"

Version-Release number of selected component:
libvirt-3.2.0-4.virtcov.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Prepare a guest xml without cache=none:
...
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/nfs/fjin/rhel7.4-GUI.img'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>

2. Do offline migration,fail:
# virsh migrate rhel7.4 qemu+ssh://10.66.144.45/system --offline --persistent
error: Unsafe migration: Migration may lead to data corruption if disks use cache != none

3. Offline migration with --unsafe, succeed
# virsh migrate rhel7.4 qemu+ssh://10.66.144.45/system --offline --persistent --unsafe

Actual results:
As step2, offline migration failed with error.

Expected results:
Offline migration should succeed

Upstream patch proposed:

https://www.redhat.com/archives/libvir-list/2017-August/msg00512.html

Upstream commit:

commit abab46a29bd9ab8ef80f86619d97bf01ed4f4e7b
Author: Pavel Hrdina <phrdina>
Date:   Thu Aug 17 18:36:46 2017 +0200

    qemu: don't check whether offline migration is safe

v3.6.0-150-gabab46a29b

Verify pass with libvirt-3.8.0-1.virtcov.el7.x86_64

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHEA-2018:0704