Re: [PATCH RFC] migration: warn about non-migratable configurations unless '--no-migration' was specified

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Eduardo Habkost (ehabkost@redhat.com) wrote:
> On Tue, Apr 20, 2021 at 12:51:43PM +0100, Dr. David Alan Gilbert wrote:
> > * Eduardo Habkost (ehabkost@redhat.com) wrote:
> > > On Mon, Apr 19, 2021 at 07:47:34PM +0100, Dr. David Alan Gilbert wrote:
> > > > * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > > > > On Mon, Apr 19, 2021 at 06:15:56PM +0100, Daniel P. Berrangé wrote:
> > > > > > On Mon, Apr 19, 2021 at 06:11:47PM +0100, Dr. David Alan Gilbert wrote:
> > > > > > > * Eduardo Habkost (ehabkost@redhat.com) wrote:
> > > > > > > > I would make live migration policy an enum, just to make sure
> > > > > > > > we are explicit about the requirements:
> > > > > > > > 
> > > > > > > > - UNKNOWN: this is the current state in QEMU 6.0, where we don't
> > > > > > > >   really know what the user expects.
> > > > > > > >   This can be the default on existing versioned machine types,
> > > > > > > >   just for compatibility.
> > > > > > > >   I suggest making this print warnings for every migration
> > > > > > > >   blocker (like this patch does).
> > > > > > > >   I suggest deprecating this behavior as soon as we can.
> > > > > > > > 
> > > > > > > > - PREFERRED: try to make the VM migratable when possible, but
> > > > > > > >   don't print a warning or error out if migration is blocked.
> > > > > > > >   This seems to be the behavior expected by libvirt today.
> > > > > > > > 
> > > > > > > > - NOT_NEEDED: live migration is not needed, and QEMU is free to
> > > > > > > >   enable features that block live migration or change guest ABI.
> > > > > > > >   We can probably make this the default on machine types that
> > > > > > > >   never supported live migration.
> > > > > > > 
> > > > > > > I suggest you could do this by adding:
> > > > > > >   -warn-none-migratable
> > > > > > >   -no-warn-none-migratable
> > > > > > > 
> > > > > > > and then argue about defaults another time.
> > > > > > 
> > > > > > If we're going to add new args, lets at least future proof our
> > > > > > approach with an extensible option that we can wire into QMP
> > > > > > too later
> > > > > > 
> > > > > >   -migratable  none|preferred|required 
> > > > > > 
> > > > > > and letting us add extra key/value pairs to tune it if desired.
> > > > > 
> > > > > Having said that, we potentially don't need a dedicated arg if we
> > > > > just make  'migratable=none|preferred|required' be a property of
> > > > > the machine type and hook everything off that
> > > > 
> > > > I think my only difficulty with that is that I don't find any of those
> > > > 3 words 'obvious'.
> > > 
> > > Any suggestions of replacements for those 3 words?
> > > 
> > > Would the descriptions below be enough to clarify their meaning
> > > in documentation?
> > 
> > I prefer things that are fairly obvious without needing to look at the
> > documentation until you want the detail.
> > 
> > > - NONE: live migration is not needed, and device or machine code
> > >   is allowed to enable features that block live migration or
> > >   change guest ABI.
> > >   (Not implemented yet)
> > > 
> > > - PREFERRED: machine and device code should try to make the VM
> > >   migratable when possible, but won't emit a warning or error out
> > >   if migration is blocked.
> > >   (Current default behavior)
> > > 
> > > - REQUIRED: live migration support is required, and adding a
> > >   migration blocker will be an error.
> > >   (Implemented today by --only-migratable)
> > 
> > How about
> >   -migratable blocked
> >      Live migration is not allowed; an outbound migration will fail
> 
> "none" and NOT_NEEDED above were about letting QEMU automatically
> enable features that block live migration or change guest ABI.
> 
> If that's implied by "blocked", I'd like to get that documented
> explicitly.  If that's not implied by "blocked", I don't
> understand what's the use case for "blocked".

My 'blocked' is stronger - migration is hard disabled by a blocker
always; it's for (rare) cases where the user wants to stop a migration
happening, even if qemu believes it can do it.

> > 
> >   -migratable allowed
> >      Live migration is allowed, but some devices/options may block
> >      it if they're unable to migrate [current default]
> 
> "preferred" above was about QEMU trying to keep live migration
> working as much as possible.  That's something we all expect QEMU
> to do, but it's not documented anywhere.
> 
> If that's implied by "allowed", I'd like to get that documented
> explicitly.  If that's not implied by "allowed", then we have a
> problem.

My difficulty by your definition is I don't understand what
'working as much as possible' means - that's the current behaviour
as I understand it.   I think mine is more explicit.

> 
> > 
> >   -migratable warn
> >       Live migration is allowed, but if some device/option is
> >       unable to migrate, migration will be blocked and a warning
> >       printed
> 
> This makes sense, but I don't think we need to support this use
> case.

I thought that was exactly what Vitaly's patch tried to do?

> > 
> >   -migratable required
> >       Live migration is allowed, attempting to add a device or
> >       enable an option that can't migrate will fail. [--only-migratable]
> 
> This matches "required" above.

Dave


> -- 
> Eduardo
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK