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

From: Alexey Kardashevskiy
Subject: Re: [Qemu-ppc] [RFC 15/17] ppc: Check that CPU model stays consistent across migration
Date: Wed, 9 Nov 2016 17:06:23 +1100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 09/11/16 15:24, David Gibson wrote:
> On Tue, Nov 08, 2016 at 05:03:49PM +1100, Alexey Kardashevskiy wrote:
>> On 08/11/16 16:29, David Gibson wrote:
>>> On Fri, Nov 04, 2016 at 06:54:48PM +1100, Alexey Kardashevskiy wrote:
>>>> 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?
>>> There probably isn't one.  But the point is it also blocks migration
>>> from a host with PVR=004B0201 (POWER8) to one with PVR=00201400
>>> (403GCX) and *that* has a clear benefit.  I don't see a way to block
>>> the second without the first, except by creating a huge compatibility
>>> matrix table, which would require inordinate amounts of time to
>>> research carefully.
>> This is pcc->pvr_match() for this purpose.
> Hmm.. thinking about this.  Obviously requiring an exactly matching
> PVR is the architecturally "safest" approach.  For TCG and PR KVM, it
> really should be sufficient - if you can select "close" PVRs at each
> end, you should be able to select exactly matching ones just as well.
> For HV KVM, we should generally be using compatibility modes to allow
> migration between a relatively wide range of CPUs.  My intention was
> basically to require moving to that model, rather than "approximate
> matching" real PVRs.

So the management stack (libvirt) will need to know that if it is HV KVM,
then -cpu host,compat=xxxx; if it is PR KVM, then -cpu XXXX and no compat.
That was really annoying when we had exact PVR matching.

> I'm still convinced using compat modes is the right way to go medium
> to long term.  However, allowing the approximate matches could make
> for a more forgiving transition, if people have existing hosts in
> "raw" mode.

Within the family, CPUs behave exactly (not slightly but exactly) the same
even though 3 of 4 bytes of the PVR value are different so enforcing PVR to
match or enforcing compatibility (which as a feature was not a great idea
from the day one) does not sound compelling.

Does x86 have anything like this compatibility thingy?

> Ok, I'll add pvr_match checking to this.


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