Re: [Qemu-devel] [PATCHv3 4/4] ppc: Rework CPU compatibility testing across migration

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

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On Thu, May 04, 2017 at 12:07:47PM +0200, Greg Kurz wrote:
> On Thu, 27 Apr 2017 17:28:43 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > Migrating between different CPU versions is a bit complicated for ppc.
> > A long time ago, we ensured 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.  That was addressed by 146c11f1 "target-ppc: Allow eventual
> > removal of old migration mistakes".
> > 
> > Now, verification of CPU compatibility across a migration basically doesn't
> > happen.  We simply ignore the PVR of the incoming migration, and hope the
> > cpu on the destination is close enough to work.
> > 
> > Now that we've cleaned up handling of processor compatibility modes for
> > pseries machine type, we can do better.  We allow migration if:
> > 
> >     * The source and destination PVRs are for the same type of CPU, as
> >       determined by CPU class's pvr_match function
> > OR  * When the source was in a compatibility mode, and the destination CPU
> >       supports the same compatibility mode
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  target/ppc/machine.c | 71 +++++++++++++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 68 insertions(+), 3 deletions(-)
> > 
> > diff --git a/target/ppc/machine.c b/target/ppc/machine.c
> > index 6cb3a48..20a46c9 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)
> >  {
> > @@ -195,6 +196,30 @@ 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;
> > @@ -203,10 +228,31 @@ 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);
> 
> As already mentioned during the review of RFCv2, this calls
> cpu_synchronize_state(CPU(cpu)) and trashes the registers.
> 
> The following changes avoid that:

This is a really ugly fix, and I think it misses the point.

If a synchronize_state() trashes state here, it means we've already
altered register state while not synchronized, which is a pre-existing
bug.

> 
> --- a/target/ppc/compat.c
> +++ b/target/ppc/compat.c
> @@ -118,7 +118,8 @@ bool ppc_check_compat(PowerPCCPU *cpu, uint32_t compat_pvr,
>      return true;
>  }
>  
> -void ppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr, Error **errp)
> +void ppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr, bool sync_needed,
> +                    Error **errp)
>  {
>      const CompatInfo *compat = compat_by_pvr(compat_pvr);
>      CPUPPCState *env = &cpu->env;
> @@ -138,7 +139,9 @@ void ppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr, Error **errp)
>          pcr = compat->pcr;
>      }
>  
> -    cpu_synchronize_state(CPU(cpu));
> +    if (sync_needed) {
> +        cpu_synchronize_state(CPU(cpu));
> +    }
>  
>      cpu->compat_pvr = compat_pvr;
>      env->spr[SPR_PCR] = pcr & pcc->pcr_mask;
> @@ -162,7 +165,7 @@ static void do_set_compat(CPUState *cs, run_on_cpu_data arg)
>      PowerPCCPU *cpu = POWERPC_CPU(cs);
>      SetCompatState *s = arg.host_ptr;
>  
> -    ppc_set_compat(cpu, s->compat_pvr, &s->err);
> +    ppc_set_compat(cpu, s->compat_pvr, true, &s->err);
>  }
>  
>  void ppc_set_compat_all(uint32_t compat_pvr, Error **errp)
> diff --git a/target/ppc/cpu.h b/target/ppc/cpu.h
> index 1d8f2fcd4a46..057785347820 100644
> --- a/target/ppc/cpu.h
> +++ b/target/ppc/cpu.h
> @@ -1364,7 +1364,8 @@ static inline int cpu_mmu_index (CPUPPCState *env, bool ifetch)
>  #if defined(TARGET_PPC64)
>  bool ppc_check_compat(PowerPCCPU *cpu, uint32_t compat_pvr,
>                        uint32_t min_compat_pvr, uint32_t max_compat_pvr);
> -void ppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr, Error **errp);
> +void ppc_set_compat(PowerPCCPU *cpu, uint32_t compat_pvr, bool sync_needed,
> +                    Error **errp);
>  #if !defined(CONFIG_USER_ONLY)
>  void ppc_set_compat_all(uint32_t compat_pvr, Error **errp);
>  #endif
> diff --git a/target/ppc/machine.c b/target/ppc/machine.c
> index 20a46c95a596..fda63532b041 100644
> --- a/target/ppc/machine.c
> +++ b/target/ppc/machine.c
> @@ -239,7 +239,7 @@ static int cpu_post_load(void *opaque, int version_id)
>      if (cpu->compat_pvr) {
>          Error *local_err = NULL;
>  
> -        ppc_set_compat(cpu, cpu->compat_pvr, &local_err);
> +        ppc_set_compat(cpu, cpu->compat_pvr, false, &local_err);
>          if (local_err) {
>              error_report_err(local_err);
>              error_free(local_err);
> 
> 
> > +        if (local_err) {
> > +            error_report_err(local_err);
> > +            error_free(local_err);
> > +            return -1;
> > +        }
> > +    } else
> > +#endif
> > +    {
> > +        if (!pvr_match(cpu, env->spr[SPR_PVR])) {
> > +            return -1;
> > +        }
> > +    }
> > +
> >      env->lr = env->spr[SPR_LR];
> >      env->ctr = env->spr[SPR_CTR];
> >      cpu_write_xer(env, env->spr[SPR_XER]);
> > @@ -560,6 +606,24 @@ static const VMStateDescription vmstate_tlbmas = {
> >      }
> >  };
> >  
> > +static bool compat_needed(void *opaque)
> > +{
> > +    PowerPCCPU *cpu = opaque;
> > +
> > +    return cpu->vhyp != NULL;
> > +}
> > +
> > +static const VMStateDescription vmstate_compat = {
> > +    .name = "cpu/compat",
> > +    .version_id = 1,
> > +    .minimum_version_id = 1,
> > +    .needed = compat_needed,
> > +    .fields = (VMStateField[]) {
> > +        VMSTATE_UINT32(compat_pvr, PowerPCCPU),
> > +        VMSTATE_END_OF_LIST()
> > +    }
> > +};
> > +
> >  const VMStateDescription vmstate_ppc_cpu = {
> >      .name = "cpu",
> >      .version_id = 5,
> > @@ -613,6 +677,7 @@ const VMStateDescription vmstate_ppc_cpu = {
> >          &vmstate_tlb6xx,
> >          &vmstate_tlbemb,
> >          &vmstate_tlbmas,
> > +        &vmstate_compat,
> >          NULL
> >      }
> >  };
> 



-- 
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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