[libvirt PATCH 06/80] qemu: Keep domain running on dst on failed post-copy migration

[Jiri Denemark]

On Wed, May 11, 2022 at 10:48:10 +0200, Peter Krempa wrote:
> On Tue, May 10, 2022 at 17:20:27 +0200, Jiri Denemark wrote:
> > There's no need to artificially pause a domain when post-copy fails. The
> > virtual CPUs may continue running, only the guest tasks that decide to
> > read a page which has not been migrated yet will get blocked.
> 
> IMO not pausing the VM is a policy decision (same way as pausing it was
> though) and should be user-configurable at migration start.
> 
> I can see that users might want to prevent a half-broken VM from
> executing until it gets attention needed to fix it, even when it's safe
> from a "theoretical" standpoint.

It depends how much was already migrated. In practise the guest may
easily stop running anyway :-) So yeah, it was a needless policy
decision which is being removed now. But the important reason behind it,
which I should have mention in the commit message is the difference
between libvirt and QEMU migration state. When libvirt connection breaks
(between daemons for p2p migration or between a client and daemons) we
consider migration as broken from the API point of view and return
failure. However, the migration may still be running just fine if the
connection between QEMU processes remains working. And since we're in
post-copy phase, the migration can even finish just fine without
libvirt. So a half-broken VM may magically become a fully working
migrated VM after our migration API reported a failure. Keeping the
domain running makes this situation easier to handle :-)

Jirka