Re: [Qemu-devel] [PATCH 1/3] target/ppc: always set PPC_MEM_TLBIE in pre 2.8 migration hack

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, May 04, 2018 at 07:54:19AM +0200, Greg Kurz wrote:
> On Fri, 4 May 2018 10:12:23 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Thu, May 03, 2018 at 11:16:29PM +0200, Greg Kurz wrote:
> > > The pseries-2.7 and older machine types require CPUPPCState::insns_flags
> > > to be strictly equal between source and destination. This checking is
> > > abusive and breaks migration of KVM guests when the host CPU models
> > > are different, even if they are compatible enough to allow the guest
> > > to run transparently. This buggy behaviour was fixed for pseries-2.8
> > > and we added some hacks to allow backward migration of older machine
> > > types. These hacks assume that the CPU belongs to the POWER8 family,
> > > which was true for most KVM based setup we cared about at the time.
> > > But now POWER9 systems are coming, and backward migration of pre 2.8
> > > guests running in POWER8 architected mode from a POWER9 host to a
> > > POWER8 host is broken:
> > > 
> > > qemu-system-ppc64: error while loading state for instance 0x0 of device
> > >  'cpu'
> > > qemu-system-ppc64: load of migration failed: Invalid argument
> > > 
> > > This happens because POWER9 doesn't set PPC_MEM_TLBIE in insns_flags,
> > > while POWER8 does. Let's force PPC_MEM_TLBIE in the migration hack to
> > > fix the issue. This is an acceptable hack because these old machine
> > > types only support CPU models that do set PPC_MEM_TLBIE.
> > > 
> > > Signed-off-by: Greg Kurz <groug@kaod.org>
> > > ---
> > >  target/ppc/machine.c |    5 +++++
> > >  1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/target/ppc/machine.c b/target/ppc/machine.c
> > > index 3d6434a006fa..ba1b9e531f97 100644
> > > --- a/target/ppc/machine.c
> > > +++ b/target/ppc/machine.c
> > > @@ -212,6 +212,11 @@ static int cpu_pre_save(void *opaque)
> > >              ;
> > >          cpu->mig_msr_mask = env->msr_mask & ~metamask;
> > >          cpu->mig_insns_flags = env->insns_flags & insns_compat_mask;
> > > +        /* CPU models supported by old machines all have PPC_MEM_TLBIE,
> > > +         * so we set it unconditionally to allow backward migration from
> > > +         * a POWER9 host to a POWER8 host.  
> > 
> > So.. the comment's slightly misleading.  This code is common to all
> > cpus, not just those used on pseries, so across that set it's not
> > really accurate.  But pseries is the only machine we support migration
> > to/from old versions with so it doesn't really matter.
> 
> Oh... I was assuming that we only supported migration for pseries
> machine, irrespective of the versions.

Actually, that's probably true.  Although there are a couple where it
might just about work.  pseries is *definitely* the only one that we
properly check and care about cross-version compatibility though.


> 
> > > +         */
> > > +        cpu->mig_insns_flags |= PPC_MEM_TLBIE;
> > >          cpu->mig_insns_flags2 = env->insns_flags2 & insns_compat_mask2;
> > >          cpu->mig_nb_BATs = env->nb_BATs;
> > >      }
> > >   
> > 
> 



-- 
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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