Re: [Qemu-devel] [PATCH 1/1] spapr: Ensure CPU cores are added contiguously and removed in LIFO order

[Bharata B Rao]

On Fri, Jul 15, 2016 at 03:29:01PM +1000, David Gibson wrote:
> On Thu, Jul 14, 2016 at 10:27:15AM +0200, Igor Mammedov wrote:
> > On Thu, 14 Jul 2016 10:51:27 +1000
> > David Gibson <address@hidden> wrote:
> > 
> > > On Wed, Jul 13, 2016 at 12:20:20PM +0530, Bharata B Rao wrote:
> > > > If CPU core addition or removal is allowed in random order leading to
> > > > holes in the core id range (and hence in the cpu_index range), migration
> > > > can fail as migration with holes in cpu_index range isn't yet handled
> > > > correctly.
> > > > 
> > > > Prevent this situation by enforcing the addition in contiguous order
> > > > and removal in LIFO order so that we never end up with holes in
> > > > cpu_index range.
> > > > 
> > > > Signed-off-by: Bharata B Rao <address@hidden>
> > > > ---
> > > > While there is work in progress to support migration when there are 
> > > > holes
> > > > in cpu_index range resulting from out-of-order plug or unplug, this 
> > > > patch
> > > > is intended as a last resort if no easy, risk-free and elegant solution
> > > > emerges before 2.7 dev cycle ends.  
> > > 
> > > Applied to ppc-for-2.7.  We can revert it once the problems with
> > > cpu_index are sorted out.
> > You'd need to add machine type specific compat option here,
> > so that new-qemu -M 2.7 wouldn't allow out of order too and
> > could be migrated to old-qemu -M 2.7
> 
> Hmm, do we care about migration from newer back to older versions of
> qemu upstream?  If so, then I guess we do need this option.  Though
> strictly we don't need it until we actually do allow any-order
> hotplug.

Right. I too thought that when we relax this restriction say in 2.8, we could
have a sPAPRMachineClass option to allow out-of-order hotplug for 2.8 upwards
and disable it for 2.7 downwards. With this, 2.7 guest started with new-qemu
can be migrated to old-qemu.

Regards,
Bharata.