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Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH] target-ppc: enable migration wit

From: Michael Mueller
Subject: Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH] target-ppc: enable migration within the same CPU family
Date: Thu, 10 Apr 2014 17:42:05 +0200

On Fri, 11 Apr 2014 01:11:10 +1000
Alexey Kardashevskiy <address@hidden> wrote:

> On 04/04/2014 11:28 PM, Alexander Graf wrote:
> > On 04/04/2014 07:17 AM, Alexey Kardashevskiy wrote:
> >> On 03/24/2014 04:28 PM, Alexey Kardashevskiy wrote:
> >>> Currently only migration fails if CPU version is different even a bit.
> >>> For example, migration from POWER7 v2.0 to POWER7 v2.1 fails because of
> >>> that. Since there is no difference between CPU versions which could
> >>> affect migration stream, we can safely enable it.
> >>>
> >>> This adds a helper to find the closest POWERPC family class (i.e. first
> >>> abstract class in hierarchy).
> >>>
> >>> This replaces VMSTATE_UINTTL_EQUAL statement with a custom handler which
> >>> checks if the source and destination CPUs belong to the same family and
> >>> fails if they are not.
> >>>
> >>> This adds a PVR reset to the default value as it will be overwritten
> >>> by VMSTATE_UINTTL_ARRAY(env.spr, PowerPCCPU, 1024).
> >>>
> >>> Since the actual migration format is not changed by this patch,
> >>> @version_id of vmstate_ppc_cpu does not have to be changed either.
> >>>
> >>> Signed-off-by: Bharata B Rao <address@hidden>
> >>> Signed-off-by: Alexey Kardashevskiy <address@hidden>
> >>
> >> Ping?
> > 
> > Can't we just always allow migration to succeed? It's a problem of the tool
> > stack above if it allows migration to an incompatible host, no?

That is the way we go with s390x. Qemu will just give hints by means of QMP 
queries what the
source model is and what models the target is capable to support. The only 
specific is, that
s390x is capable to limit the target cpu model. That means although the host is 
a 2827-GA2, the
KVM might look like an older model (2817-GA2). That simplifies the 
implementation in the above
management stack to a simple string compare without the need of model specific 
knowledge in terms
of compatibility.

> After very convincing mail from Peter Maydell, this is the only solution,
> correct? If so, I'll make a patch then.

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