Re: [Qemu-devel] [Qemu-ppc] [RFCv2 12/12] ppc: Rework CPU compatibility testing across migration

[Greg Kurz <groug@kaod.org>]

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On Mon, 5 Dec 2016 15:09:16 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Fri, Dec 02, 2016 at 03:48:25PM +0100, Greg Kurz wrote:
> > On Wed, 16 Nov 2016 09:17:55 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > Migrating between different CPU versions is quite complicated for ppc.
> > > A long time ago, we ensure identical CPU versions at either end by checking
> > > the PVR had the same value.  However, this breaks under KVM HV, because
> > > we always have to use the host's PVR - it's not virtualized.  That would
> > > mean we couldn't migrate between hosts with different PVRs, even if the
> > > CPUs are close enough to compatible in practice (sometimes identical cores
> > > with different surrounding logic have different PVRs, so this happens in
> > > practice quite often).
> > > 
> > > So, we removed the PVR check, but instead checked that several flags
> > > indicating supported instructions matched.  This turns out to be a bad
> > > idea, because those instruction masks are not architected information, but
> > > essentially a TCG implementation detail.  So changes to qemu internal CPU
> > > modelling can break migration - this happened between qemu-2.6 and
> > > qemu-2.7.
> > > 
> > > Modern server-class CPUs can be placed into compatibility modes.  Now that
> > > we're handling those properly, we finally have the information to sanely
> > > deal with CPU compatibility across migration.
> > > 
> > > This patch bumps the migration version number for the ppc CPU removing the
> > > instruction mask field (and some other unwise VMSTATE_EQUAL checks), and
> > > adding the compatibility PVR to the migration stream.
> > >   
> > 
> > Things have changed since you posted this RFC:
> > 
> > commit 16a2497bd44cac1856e259654fd304079bd1dcdc
> > Author: David Gibson <david@gibson.dropbear.id.au>
> > Date:   Mon Nov 21 16:28:12 2016 +1100
> > 
> >     target-ppc: Fix CPU migration from qemu-2.6 <-> later versions
> > 
> > and
> > 
> > commit 146c11f16f12dbfea62cbd7f865614bb6fcbc6b5
> > Author: David Gibson <david@gibson.dropbear.id.au>
> > Date:   Mon Nov 21 16:29:30 2016 +1100
> > 
> >     target-ppc: Allow eventual removal of old migration mistakes
> > 
> > I guess that the version bumping isn't necessary anymore if we keep these.
> > 
> > I'll assume yes and rebase this patch against current master, simply dropping
> > the version bumping and related lines.  
> 
> Yeah, I realised that breaking backwards migration was a bad idea, and
> with some help from Dave Gilbert worked out how to make it possible.
> 
> I realize I'm going to have to rework my compat series in light of
> these changes.
> 

While doing so, maybe you can fold the following in your patch, and thus
make tests/qom-test happy:

--- a/target-ppc/translate_init.c
+++ b/target-ppc/translate_init.c
@@ -32,6 +32,7 @@
 #include "qapi/visitor.h"
 #include "hw/qdev-properties.h"
 #include "hw/ppc/ppc.h"
+#include "sysemu/qtest.h"
 
 //#define PPC_DUMP_CPU
 //#define PPC_DEBUG_SPR
@@ -8442,7 +8443,9 @@ POWERPC_FAMILY(POWER5P)(ObjectClass *oc, void *data)
 static void getset_compat_deprecated(Object *obj, Visitor *v, const char *name,
                                      void *opaque, Error **errp)
 {
-    error_report("CPU 'compat' property is deprecated and has no effect; use max-cpu-compat machine property instead");
+    if (!qtest_enabled()) {
+        error_report("CPU 'compat' property is deprecated and has no effect; use max-cpu-compat machine property instead");
+    }
     visit_type_null(v, name, errp);
 }
 
Also a friendly reminder: cpu registers get trashed during cpu post
load (see at the end of this mail).


Cheers.

--
Greg

> > > We consider the CPUs compatible for migration if:
> > >     * The source was running in a compatibility mode which the destination
> > >       supports
> > > OR  * The source has a PVR matching the same qemu CPU class as the
> > >       destination, either an exact match or an approximate match determined
> > >       by the cpu class's pvr_match hook.
> > > 
> > > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > > ---
> > >  target-ppc/machine.c | 87 +++++++++++++++++++++++++++++++++++++++++++++++-----
> > >  1 file changed, 79 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/target-ppc/machine.c b/target-ppc/machine.c
> > > index e43cb6c..25a30d5 100644
> > > --- a/target-ppc/machine.c
> > > +++ b/target-ppc/machine.c
> > > @@ -8,6 +8,7 @@
> > >  #include "helper_regs.h"
> > >  #include "mmu-hash64.h"
> > >  #include "migration/cpu.h"
> > > +#include "qapi/error.h"
> > >  
> > >  static int cpu_load_old(QEMUFile *f, void *opaque, int version_id)
> > >  {
> > > @@ -163,6 +164,30 @@ static void cpu_pre_save(void *opaque)
> > >      }
> > >  }
> > >  
> > > +/*
> > > + * Determine if a given PVR is a "close enough" match to the CPU
> > > + * object.  For TCG and KVM PR it would probably be sufficient to
> > > + * require an exact PVR match.  However for KVM HV the user is
> > > + * restricted to a PVR exactly matching the host CPU.  The correct way
> > > + * to handle this is to put the guest into an architected
> > > + * compatibility mode.  However, to allow a more forgiving transition
> > > + * and migration from before this was widely done, we allow migration
> > > + * between sufficiently similar PVRs, as determined by the CPU class's
> > > + * pvr_match() hook.
> > > + */
> > > +static bool pvr_match(PowerPCCPU *cpu, uint32_t pvr)
> > > +{
> > > +    PowerPCCPUClass *pcc = POWERPC_CPU_GET_CLASS(cpu);
> > > +
> > > +    if (pvr == pcc->pvr) {
> > > +        return true;
> > > +    }
> > > +    if (pcc->pvr_match) {
> > > +        return pcc->pvr_match(pcc, pvr);
> > > +    }
> > > +    return false;
> > > +}
> > > +
> > >  static int cpu_post_load(void *opaque, int version_id)
> > >  {
> > >      PowerPCCPU *cpu = opaque;
> > > @@ -171,10 +196,31 @@ 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) {
> > > +        Error *local_err = NULL;
> > > +
> > > +        ppc_set_compat(cpu, cpu->compat_pvr, &local_err);  
> > 
> > This calls cpu_synchronize_state(CPU(cpu)) and trashes the registers. This
> > is the root cause behind the program interrupts I mentioned in another mail.
> > 
> > Adding a sync_needed boolean argument to ppc_set_compat() seems to be enough
> > to get this working. So I'll just do that and rerun the tests.
> > 
> > Cheers.
> >   
> 


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