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 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

  -migratable allowed
     Live migration is allowed, but some devices/options may block
     it if they're unable to migrate [current default]

  -migratable warn
      Live migration is allowed, but if some device/option is
      unable to migrate, migration will be blocked and a warning
      printed

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

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