Re: [Qemu-devel] [PATCH 1/1] kvm: support TSC deadline MSR

[Anthony Liguori]

On 10/10/2011 09:58 AM, Avi Kivity wrote:
> On 10/10/2011 04:54 PM, Anthony Liguori wrote:
> > On 10/04/2011 05:20 PM, Marcelo Tosatti wrote:
> > > On Tue, Oct 04, 2011 at 07:53:42PM +0200, Avi Kivity wrote:
> > > > On 10/03/2011 10:55 PM, Marcelo Tosatti wrote:
> > > > > From: "Liu, Jinsong"<address@hidden>
> > > > >
> > > > > KVM add emulation of lapic tsc deadline timer for guest.
> > > > > This patch is co-operation work at qemu side.
> > > > >
> > > > > -#define CPU_SAVE_VERSION 12
> > > > > +#define CPU_SAVE_VERSION 13
> > > >
> > > > Unfortunate. Can't we use subsections?
> > >
> > > Yes, i'll look into it tomorrow.
> >
> > Subsections are still broken at the moment although Juan has some patches.
> > Bumping the version is the safe thing to do.
>
> It's irreversible, once we release a version with a bumped ID we can't go back.

But the question is whether we've bumped *any* versions of common devices since 
0.15 because if so, it's moot here.  Once any device bumps a version id, 
migration is incompatible.

Subsections are nice for stable branches, but they don't solve inter-version 
compatibility.  Most importantly, subsections are broken today so until we 
straighten things out there, we can't rely on them.

Regards,

Anthony Liguori