Re: [Qemu-devel] [RFC 15/17] ppc: Check that CPU model stays consistent across migration

[Alexey Kardashevskiy]

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 <address@hidden>
> ---
>  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?


>  
>      env->lr = env->spr[SPR_LR];
>      env->ctr = env->spr[SPR_CTR];
> 


-- 
Alexey