Re: [Qemu-devel] [PATCH] target-ppc: Add POWER8E_v2.1 CPU model.

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 07/09/2015 02:35 AM, Andrea Bolognani wrote:
> On Wed, 2015-07-08 at 17:02 +1000, Alexey Kardashevskiy wrote:
>>
>>>>> I think it's because libvirt likes to specify a specific CPU -
>>>>> and if
>>>>> it gets the new PVR from the host, qemu won't understand it.
>>>>
>>>> A specific CPU in this case is "POWER8", I added this
>>>> specifically for
>>>> libvirt (to allow migration between all versions of POWER8), it
>>>> should not
>>>> use versioned CPUs and it does not in powerkvm.
>
> Does "all versions of POWER8" include things like POWER8E, POWER8NVL
> and "POWER8 DD1", as one of the variations is known in the kernel
> source? Can we safely migrate guests eg. from a POWER8 v1.0 host to
> a POWER8E v2.1 host?

Yes. afaik the only difference between POWER8 and POWER8E is how many cores 
are packed into an actual chip.


>>> Uh.. won't that make qemu attempt to set a specific PVR, though -
>>> which will fail with recent KVM if it's not *exactly* the same as
>>> the
>>> host PVR.
>>
>> The "POWER8" CPU class is created dynamically (like the "host" CPU
>> class)
>> and has the actual host PVR so setting it to KVM cannot fail.
>>
>> When TCG, the class is not registered and alias is used instead.
>
> Can you please point out where the dynamical creation happens for the
> POWER8 class? I haven't been able to locate it, but then again I'm
> all but familiar with QEMU's source :)


kvm_ppc_register_host_cpu_type(), last chunk:

/* Register generic family CPU class for a family */
pvr_pcc = ppc_cpu_get_family_class(pvr_pcc);
dc = DEVICE_CLASS(pvr_pcc);
type_info.parent = object_class_get_name(OBJECT_CLASS(pvr_pcc));
type_info.name = g_strdup_printf("%s-"TYPE_POWERPC_CPU, dc->desc);
type_register(&type_info);


>  From libvirt's point of view, it would be nice to be able to identify
> as "POWER8" anything that looks like it, by matching the host's PVR
> agains a numer of known PVRs (with relative bitmask). Ideally, if the
> host can be identified as POWER8, that's the only CPU model libvirt
> should advertise...

I have a very little idea about libvirt here. QEMU considers everything 
with 0x004dxxxx and 0x004bxxxx as POWER8 (ppc_pvr_match_power8() helper) 
and supports migration between these.

I am adding Shiva to the coversation, he might enlighen us how this is 
solved by powerkvm's libvirt.


-- 
Alexey