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Re: [Qemu-devel] [RFC 12/17] ppc: Migrate compatibility mode


From: David Gibson
Subject: Re: [Qemu-devel] [RFC 12/17] ppc: Migrate compatibility mode
Date: Tue, 8 Nov 2016 16:19:50 +1100
User-agent: Mutt/1.7.1 (2016-10-04)

On Fri, Nov 04, 2016 at 04:58:47PM +1100, Alexey Kardashevskiy wrote:
> On 30/10/16 22:12, David Gibson wrote:
> > Server-class POWER CPUs can be put into several compatibility modes.  These
> > can be specified on the command line, or negotiated by the guest during
> > boot.
> > 
> > Currently we don't migrate the compatibility mode, which means after a
> > migration the guest will revert to running with whatever compatibility
> > mode (or none) specified on the command line.
> > 
> > With the limited range of CPUs currently used, this doesn't usually cause
> > a problem, but it could.  Fix this by adding the compatibility mode (if
> > set) to the migration stream.
> > 
> > Signed-off-by: David Gibson <address@hidden>
> > ---
> >  target-ppc/machine.c | 34 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 34 insertions(+)
> > 
> > diff --git a/target-ppc/machine.c b/target-ppc/machine.c
> > index 4820f22..5d87ff6 100644
> > --- a/target-ppc/machine.c
> > +++ b/target-ppc/machine.c
> > @@ -9,6 +9,7 @@
> >  #include "mmu-hash64.h"
> >  #include "migration/cpu.h"
> >  #include "exec/exec-all.h"
> > +#include "qapi/error.h"
> >  
> >  static int cpu_load_old(QEMUFile *f, void *opaque, int version_id)
> >  {
> > @@ -176,6 +177,20 @@ static int cpu_post_load(void *opaque, int version_id)
> >       * software has to take care of running QEMU in a compatible mode.
> >       */
> >      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);
> > +        if (local_err) {
> > +            error_report_err(local_err);
> > +            error_free(local_err);
> > +            return -1;
> > +        }
> > +    }
> > +#endif
> > +
> >      env->lr = env->spr[SPR_LR];
> >      env->ctr = env->spr[SPR_CTR];
> >      cpu_write_xer(env, env->spr[SPR_XER]);
> > @@ -528,6 +543,24 @@ static const VMStateDescription vmstate_tlbmas = {
> >      }
> >  };
> >  
> > +static bool compat_needed(void *opaque)
> > +{
> > +    PowerPCCPU *cpu = opaque;
> > +
> > +    return cpu->compat_pvr != 0;
> 
> 
> Finally got to trying how this affects migration :)
> 
> This breaks migration to QEMU <=2.7, and it should not at least when both
> source and destination are running with  -cpu host,compat=power7.

IIUC, we don't generally try to maintain backwards migration, even for
old machine types.

> 
> 
> > +}
> > +
> > +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,
> > @@ -580,6 +613,7 @@ 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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