[libvirt] [PATCH] qemu: Fix post-copy migration on the source

[Ján Tomko]

On Fri, Nov 16, 2018 at 02:16:33PM +0100, Jiri Denemark wrote:
>Post-copy migration has been broken on the source since commit
>v3.8.0-245-g32c29f10db which implemented support for
>pause-before-switchover QEMU migration capability.
>
>Even though the migration itself went well, the source did not really
>know when it switched to the post-copy mode despite the messages logged
>by MIGRATION event handler. As a result of this, the events emitted by
>source libvirtd were not accurate and statistics of the completed
>migration would cover only the pre-copy part of migration. Moreover, if
>migration failed during the post-copy phase for some reason, the source
>libvirtd would just happily resume the domain, which could lead to disk
>corruption.
>
>With the pause-before-switchover capability enabled, the order of events
>emitted by QEMU changed:
>
>                    pause-before-switchover
>           disabled                        enabled
>    MIGRATION, potcopy-active       STOP

s/pot/post/

But I guess it's just a matter of time until someone invents a
virtualization technology using plants, pots and gardening for the
terminology.

>    STOP                            MIGRATION, pre-switchover
>                                    MIGRATION, postcopy-active
>
>The STOP even handler checks the migration status (postcopy-active) and
>sets the domain state accordingly. Which is sufficient when
>pause-before-switchover is disabled, but once we enable it, the
>migration status is still active when we get STOP from QEMU. Thus the
>domain state set in the STOP handler has to be corrected once we are
>notified that migration changed to postcopy-active.
>
>This results in two SUSPENDED events to be emitted by the source
>libvirtd during post-copy migration. The first one with
>VIR_DOMAIN_EVENT_SUSPENDED_MIGRATED detail, while the second one reports
>the corrected VIR_DOMAIN_EVENT_SUSPENDED_POSTCOPY detail. This is
>inevitable because we don't know whether migration will eventually
>switch to post-copy at the time we emit the first event.
>
>https://bugzilla.redhat.com/show_bug.cgi?id=1647365
>
>Signed-off-by: Jiri Denemark <jdenemar at redhat.com>
>---
> src/qemu/qemu_process.c | 26 +++++++++++++++++++++++++-
> 1 file changed, 25 insertions(+), 1 deletion(-)
>

Reviewed-by: Ján Tomko <jtomko at redhat.com>

Jano
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