Re: [Qemu-devel] [RFC 15/17] ppc: Check that CPU model stays consistent across migration

[David Gibson]

On Fri, Nov 04, 2016 at 06:54:48PM +1100, Alexey Kardashevskiy wrote:
> On 30/10/16 22:12, David Gibson wrote:
> > When a vmstate for the ppc cpu was first introduced (a90db15 "target-ppc:
> > Convert ppc cpu savevm to VMStateDescription"), a VMSTATE_EQUAL was used
> > to ensure that identical CPU models were used at source and destination
> > as based on the PVR (Processor Version Register).
> > 
> > However this was a problem for HV KVM, where due to hardware limitations
> > we always need to use the real PVR of the host CPU.  So, to allow
> > migration between hosts with "similar enough" CPUs, the PVR check was
> > removed in 569be9f0 "target-ppc: Remove PVR check from migration".  This
> > left the onus on user / management to only attempt migration between
> > compatible CPUs.
> > 
> > Now that we've reworked the handling of compatiblity modes, we have the
> > information to actually determine if we're making a compatible migration.
> > So this patch partially restores the PVR check.  If the source was running
> > in a compatibility mode, we just make sure that the destination cpu can
> > also run in that compatibility mode.  However, if the source was running
> > in "raw" mode, we verify that the destination has the same PVR value.
> > 
> > Signed-off-by: David Gibson <address@hidden>
> > ---
> >  target-ppc/machine.c | 15 +++++++++++----
> >  1 file changed, 11 insertions(+), 4 deletions(-)
> > 
> > diff --git a/target-ppc/machine.c b/target-ppc/machine.c
> > index 5d87ff6..62b9e94 100644
> > --- a/target-ppc/machine.c
> > +++ b/target-ppc/machine.c
> > @@ -173,10 +173,12 @@ static int cpu_post_load(void *opaque, int version_id)
> >      target_ulong msr;
> >  
> >      /*
> > -     * We always ignore the source PVR. The user or management
> > -     * software has to take care of running QEMU in a compatible mode.
> > +     * If we're operating in compat mode, we should be ok as long as
> > +     * the destination supports the same compatiblity mode.
> > +     *
> > +     * Otherwise, however, we require that the destination has exactly
> > +     * the same CPU model as the source.
> >       */
> > -    env->spr[SPR_PVR] = env->spr_cb[SPR_PVR].default_value;
> >  
> >  #if defined(TARGET_PPC64)
> >      if (cpu->compat_pvr) {
> > @@ -188,8 +190,13 @@ static int cpu_post_load(void *opaque, int version_id)
> >              error_free(local_err);
> >              return -1;
> >          }
> > -    }
> > +    } else
> >  #endif
> > +    {
> > +        if (env->spr[SPR_PVR] != env->spr_cb[SPR_PVR].default_value) {
> > +            return -1;
> > +        }
> > +    }
> 
> This should break migration from host with PVR=004d0200 to host with
> PVR=004d0201, what is the benefit of such limitation?

There probably isn't one.  But the point is it also blocks migration
from a host with PVR=004B0201 (POWER8) to one with PVR=00201400
(403GCX) and *that* has a clear benefit.  I don't see a way to block
the second without the first, except by creating a huge compatibility
matrix table, which would require inordinate amounts of time to
research carefully.

-- 
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

signature.asc
Description: PGP signature