Re: [PATCH] vhost-user-fs: add capability to allow migration

[Michael S. Tsirkin]

On Sun, Jan 22, 2023 at 02:36:04PM +0200, Anton Kuchin wrote:
> > > This flag should be set when qemu don't need to worry about any
> > > external state stored in vhost-user daemons during migration:
> > > don't fail migration, just pack generic virtio device states to
> > > migration stream and orchestrator guarantees that the rest of the
> > > state will be present at the destination to restore full context and
> > > continue running.
> > Sorry  I still do not get it.  So fundamentally, why do we need this 
> > property?
> > vhost-user-fs is not created by default that we'd then need opt-in to
> > the special "migrateable" case.
> > That's why I said it might make some sense as a device property as qemu
> > tracks whether device is unplugged for us.
> > 
> > But as written, if you are going to teach the orchestrator about
> > vhost-user-fs and its special needs, just teach it when to migrate and
> > where not to migrate.
> > 
> > Either we describe the special situation to qemu and let qemu
> > make an intelligent decision whether to allow migration,
> > or we trust the orchestrator. And if it's the latter, then 'migrate'
> > already says orchestrator decided to migrate.
> 
> The problem I'm trying to solve is that most of vhost-user devices
> now block migration in qemu. And this makes sense since qemu can't
> extract and transfer backend daemon state. But this prevents us from
> updating qemu executable via local migration. So this flag is
> intended more as a safety check that says "I know what I'm doing".
> 
> I agree that it is not really necessary if we trust the orchestrator
> to request migration only when the migration can be performed in a
> safe way. But changing the current behavior of vhost-user-fs from
> "always blocks migration" to "migrates partial state whenever
> orchestrator requests it" seems a little  dangerous and can be
> misinterpreted as full support for migration in all cases.

It's not really different from block is it? orchestrator has to arrange
for backend migration. I think we just assumed there's no use-case where
this is practical for vhost-user-fs so we blocked it.
But in any case it's orchestrator's responsibility.

-- 
MST