Re: [PATCH v2 01/27] migration: Network Failover can't work with a paused guest

[Daniel P . Berrangé]

On Wed, Dec 02, 2020 at 06:19:29AM -0500, Michael S. Tsirkin wrote:
> On Wed, Dec 02, 2020 at 10:55:15AM +0000, Daniel P. Berrangé wrote:
> > On Wed, Dec 02, 2020 at 11:51:05AM +0100, Juan Quintela wrote:
> > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > > On Wed, Dec 02, 2020 at 05:31:53AM -0500, Michael S. Tsirkin wrote:
> > > >> On Wed, Dec 02, 2020 at 10:27:18AM +0000, Daniel P. Berrangé wrote:
> > > >> > On Wed, Dec 02, 2020 at 05:13:18AM -0500, Michael S. Tsirkin wrote:
> > > >> > > On Wed, Nov 18, 2020 at 09:37:22AM +0100, Juan Quintela wrote:
> > > >> > > > If we have a paused guest, it can't unplug the network VF 
> > > >> > > > device, so
> > > >> > > > we wait there forever.  Just change the code to give one error 
> > > >> > > > on that
> > > >> > > > case.
> > > >> > > > 
> > > >> > > > Signed-off-by: Juan Quintela <quintela@redhat.com>
> > > >> > > 
> > > >> > > It's certainly possible but it's management that created
> > > >> > > this situation after all - why do we bother to enforce
> > > >> > > a policy? It is possible that management will unpause immediately
> > > >> > > afterwards and everything will proceed smoothly.
> > > >> > > 
> > > >> > > Yes migration will not happen until guest is
> > > >> > > unpaused but the same it true of e.g. a guest that is stuck
> > > >> > > because of a bug.
> > > >> > 
> > > >> > That's pretty different behaviour from how migration normally handles
> > > >> > a paused guest, which is that it is guaranteed to complete the 
> > > >> > migration
> > > >> > in as short a time as network bandwidth allows.
> > > >> > 
> > > >> > Just ignoring the situation I think will lead to surprise apps / 
> > > >> > admins,
> > > >> > because the person/entity invoking the migration is not likely to 
> > > >> > have
> > > >> > checked wether this particular guest uses net failover or not before
> > > >> > invoking - they'll just be expecting a paused migration to run fast 
> > > >> > and
> > > >> > be guaranteed to complete.
> > > >> > 
> > > >> > Regards,
> > > >> > Daniel
> > > >> 
> > > >> Okay I guess. But then shouldn't we handle the reverse situation too:
> > > >> pausing guest after migration started but before device was
> > > >> unplugged?
> > > >> 
> > > >
> > > > Thinking of which, I have no idea how we'd handle it - fail
> > > > pausing guest until migration is cancelled?
> > > >
> > > > All this seems heavy handed to me ...
> > > 
> > > This is the minimal fix that I can think of.
> > > 
> > > Further solution would be:
> > > - Add a new migration parameter: migrate-paused
> > > - change libvirt to use the new parameter if it exist
> > > - in qemu, when we do start migration (but after we wait for the unplug
> > >   device) paused the guest before starting migration and resume it after
> > >   migration finish.
> > 
> > It would also have to handle issuing of paused after migration has
> > been started - delay the pause request until the nuplug is complete
> > is one answer.
> 
> Hmm my worry would be that pausing is one way to give cpu
> resources back to host. It's problematic if guest can delay
> that indefinitely.

hmm, yes, that is awkward.  Perhaps we should just report an explicit
error then. In normal cases this won't happen, as unplug will have
easily completed before the mgmt app pauses the running migration.
In broken/malicious guest cases, this at least ives mgmt a heads up
that something is wrong and they might then decide to cancel the
migration.


Regards,
Daniel
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