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

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

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On Tue, Nov 08, 2016 at 04:51:10PM +1100, Alexey Kardashevskiy wrote:
> On 08/11/16 16:19, David Gibson wrote:
> > 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 <david@gibson.dropbear.id.au>
> >>> ---
> >>>  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.
> 
> 
> I thought the opposite - we generally try to maintain it, this is pretty
> much why we use these subsections in cases like this; otherwise you could
> just add a new field and bump the vmstate_ppc_cpu.version.

Hm, I guess that's true.  We do at least try to allow backwards
migration, we just don't insist on it.  The example here partially
suceeds - it will allow backwards migration for CPUs in raw mode.

I'll look at just bumping the version instead.

> 
> 
> > 
> >>
> >>
> >>> +}
> >>> +
> >>> +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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