Re: [PATCH] migration: Provide a test for migratability

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

* Eric Blake (eblake@redhat.com) wrote:
> On 1/21/21 12:51 PM, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Provide a simple way to see if there's currently a migration blocker in
> > operation:
> > 
> > $ ./x86_64-softmmu/qemu-system-x86_64 -nographic -M pc,usb=on -chardev null,id=n -device usb-serial,chardev=n
> > 
> > (qemu) info migratable
> > Error: State blocked by non-migratable device '0000:00:01.2/1/usb-serial'
> > 
> > $ ./x86_64-softmmu/qemu-system-x86_64 -nographic
> > 
> > (qemu) info migratable
> > Migratable
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> 
> 
> > +++ b/qapi/migration.json
> > @@ -366,6 +366,20 @@
> >  ##
> >  { 'command': 'query-migrate', 'returns': 'MigrationInfo' }
> >  
> > +##
> > +# @query-migratable:
> > +# Tests whether it will be possible to migrate the VM in the current state.
> > +#
> > +# Returns: nothing on success (i.e. if the VM is migratable)
> 
> Do we really need a new command?  Or can we get away with enhancing the
> existing 'query-migrate' to add another bool field to 'MigrationInfo'?

In the case where the VM is not migratable this command errors with the
message set in a migration-blocker (Our migration blocker mechanism
always takes an Error* as the reason).

It didn't seem right to make that change for the existing commands;
so how would you like the existing MigrationInfo to change; a boolean to
say yes/no, and an error/list of errors for the reason?

Dave

> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK