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

[Daniel P . Berrangé]

On Wed, Dec 02, 2020 at 06:37:46AM -0500, Michael S. Tsirkin wrote:
> On Wed, Dec 02, 2020 at 11:26:39AM +0000, Daniel P. Berrangé wrote:
> > 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.
> 
> Report an error in response to which command? Do you mean
> fail migration?

If mgt attempt to pause an existing migration that hasn't finished
the PCI unplug stage, then fail the pause request.

> 
> > 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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