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

[Eduardo Habkost <ehabkost@redhat.com>]

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".


> 
>   -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.


> 
>   -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.

> 
>   -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.

-- 
Eduardo