Re: [Qemu-devel] [PATCH v4 5/6] xics: directly register ICPState objects to vmstate

[David Gibson]

On Tue, Jun 13, 2017 at 09:14:55AM +0200, Greg Kurz wrote:
> On Mon, 12 Jun 2017 22:15:46 +0800
> David Gibson <address@hidden> wrote:
> 
> > On Thu, Jun 08, 2017 at 03:43:18PM +0200, Greg Kurz wrote:
> > > The ICPState objects are currently registered to vmstate as qdev objects.
> > > Their instance ids are hence computed automatically in the migration code,
> > > and thus depends on the order the CPU cores were plugged.
> > > 
> > > If the destination had its CPU cores plugged in a different order than the
> > > source, then ICPState objects will have different instance_ids and load
> > > the wrong state.
> > > 
> > > Since CPU objects have a reliable cpu_index which is already used as
> > > instance_id in vmstate, let's use it for ICPState as well.
> > > 
> > > Signed-off-by: Greg Kurz <address@hidden>  
> > 
> > This is certainly an improvement.  You answered my query on the
> > previous version as to why this doesn't break migration, but that
> > information should go into the commit message.
> > 
> 
> I'll add this explanation to the changelog.
> 
> > So, ideally, we would use the XICS "server number" as the migration
> > key.  That's an architected part of the XICs state, since those values
> > are entered explicitly into the ICS.  We have a way to go from server
> > number to ICP at the moment, but not the reverse, but we can fix that.
> > 
> > Unfortunately I think those won't always match existing automatically
> > generated IDs, which makes things harder.
> > 
> 
> Maybe things will get better when cpu_dt_id can be contiguous:
> 
> http://www.spinics.net/lists/kvm/msg150390.html
> 
> But I guess it won't help with existing machine types.

Right.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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