Re: [PATCH v4 08/22] KVM: arm64: ITS: Implement vgic_mmio_uaccess_write_its_iidr

[Peter Maydell <peter.maydell@linaro.org>]

On 11 April 2017 at 11:29, Marc Zyngier <marc.zyngier@arm.com> wrote:
> On 11/04/17 11:16, Peter Maydell wrote:
>> On 11 April 2017 at 11:08, Auger Eric <eric.auger@redhat.com> wrote:
>>> On 11/04/2017 12:05, Marc Zyngier wrote:
>>>> On 10/04/17 16:17, Auger Eric wrote:
>>>>> A (v1) -> B (v1 & v2): migration OK
>>>>> B (v1 & v2) -> C (v1): migration NOK
>>>>
>>>> So what does IIDR report on B once the A->B migration has taken place?
>>>> Does it report v2?
>>>
>>> yes that was the plan
>>
>> Hmm, the IIDR value shouldn't change across a migration I think.
>> It's guest visible so the guest should see it still the same
>> value even after migration.
>
> That's my worry. But we then have a problem when this VM migrates again.
> How does userspace find out which ABI to use then?

Userspace shouldn't need to care; the idea of this scheme is
that if the kernel has to change its ABI then the kernel
is what has to deal with the change.

I think that means that if you can read the v1 format on input
you have to output it too, though.

thanks
-- PMM