Re: [Qemu-devel] [RFC PATCH] target-ppc: enable migration within the same CPU family

[Alexey Kardashevskiy]

On 04/08/2014 08:32 PM, Michael Mueller wrote:
> On Tue, 08 Apr 2014 20:04:42 +1000
> Alexey Kardashevskiy <address@hidden> wrote:
> 
>> On 04/08/2014 07:47 PM, Michael Mueller wrote:
>>> On Tue, 08 Apr 2014 11:23:14 +1000
>>> Alexey Kardashevskiy <address@hidden> wrote:
>>>
>>>> On 04/08/2014 04:53 AM, Andreas Färber wrote:
>>>>> Am 07.04.2014 05:27, schrieb Alexey Kardashevskiy:
>>>>>> 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?
>>>>>>
>>>>>> This is not how libvirt works. It simply sends the source XML, 
>>>>>> reconstructs
>>>>>> a guest on the destination side and then migrates. hoping that the
>>>>>> migration will fail is something (which only QEMU has knowledge of) is
>>>>>> incompatible. The new guest will start with "-cpu host" (as the source) 
>>>>>> but
>>>>>> it will create diffrent CPU class and do different things. If we do not
>>>>>> check PVR (and cpu_dt_id and chip_id - the latter is coming soon) and
>>>>>> migrate power8->power7, we can easily get a broken guest.
>>>>>
>>>>> The response is very simple: -cpu host is not supported for migration.
>>>>> Same as for x86 hosts.
>>>>
>>>> Is there any good reason to limit ourselves on POWERPC?
>>>>
>>>>> As you say, the domain config is transferred by libvirt:
>>>>> If you use -cpu POWER7, you can migrate from POWER7 to POWER8 and back;
>>>>> if you use -cpu POWER8, you can only migrate on POWER8.
>>>>
>>>> -cpu other that "host" is not supported by HV KVM, only "compat" which
>>>> upstream QEMU does not have yet. So you are saying that the migration is
>>>> not supported by upstream QEMU for at least SPAPR. Well, ok, it is dead
>>>> anyway so I am fine :)
>>>>
>>>
>>> With s390x we have a similar situation. Thus we came up with a mechanism to 
>>> limit
>>> the CPU functionality of a possible target system. Our patch implements CPU 
>>> models
>>> based on TYPE and GA like 2817-ga1, etc. (GA represents a CPU facility set 
>>> and an IBC
>>> value (Instruction Blocking Control, reduces the instruction set to the 
>>> requested
>>> level)) When a guest is started, it receives its CPU model by means of 
>>> option -cpu.
>>> "host" equates the configuration of the current system. We implemented 
>>> "query-cpu-model"
>>> returning the actual model, here maybe { name: "2817-ga1" }. To find a 
>>> suitable
>>> migration target in a remote CEC, libvirt has to "query-cpu-definitions" 
>>> returning a
>>> list of models supported by the target system "{{name: "2827-ga2"}, {name: 
>>> "2827-ga1"},
>>> {name: "2817-ga2"},...]. A match means the system is suitable and can be 
>>> used
>>> as migration target.
>>
>> Sorry, I do not follow you. You hacked libvirt to run the destination QEMU
>> with a specific CPU model? Or it is in QEMU? Where? What I see now is this:
>>
>> static const VMStateDescription vmstate_s390_cpu = {
>>     .name = "cpu",
>>     .unmigratable = 1,
>> };
>>
>> Does not look like it supports migration :) Thanks!
>>
> 
> The code you're missing is not upstream yet. The s390x guest can be migrated 
> in the meantime.
> Yes, libvirt currently gets an extension to be able to identify and startup 
> suitable migration
> targets for s390x on behalf of the mentioned qemu cpu model. BTW can you 
> point me to the above
> mentioned SPAPR stuff...


Mmm. What stuff? :) At the moment POWERPC guests migrate if PVR (processor
version register) value is exactly the same. I am trying to relax this
limitation to any version within same CPU family, like power7 v1.0 and v2.1.



-- 
Alexey