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

[Alexey Kardashevskiy]

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!


-- 
Alexey