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 <da...@gibson.dropbear.id.au>
> > ---
> > 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
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