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

[Marc Zyngier <marc.zyngier@arm.com>]

On 10/04/17 16:17, Auger Eric wrote:
> Hi Marc,
> 
> On 10/04/2017 16:57, Marc Zyngier wrote:
>> On 10/04/17 15:32, Auger Eric wrote:
>>> Hi Marc,
>>>
>>> On 08/04/2017 12:42, Marc Zyngier wrote:
>>>> On Mon, Mar 27 2017 at 10:30:58 AM, Eric Auger <eric.auger@redhat.com> wrote:
>>>>> The GITS_IIDR revision field is used to encode the version of the
>>>>> table layout (ABI). So we need to restore it to check the table
>>>>> layout to be restored is compatible with the destination vITS.
>>>>>
>>>>> The user selected revision is stored in the user_revision field.
>>>>> It will be compared against the REV num at table restoration time.
>>>>
>>>> Why isn't it sufficient to keep it GITS_IIDR RO and let userspace find
>>>> out about the ABI revision that the kernel understands?
>>>>
>>>> Or are we planning on supporting multiple ABIs in the kernel?
>>> Yes as discussed with Peter, the plan is to allow several ABIs. So the
>>> userspace informs the destination about the ABI revision of the stored
>>> tables (contained by the GITS_IIDR). If the destination KVM does not
>>> support this ABI revision, table restore will fail.
>>>  If so, do
>>>> we have a deprecation policy/plan? I don't mind either way, but it would
>>>> be good to document it...
>>>>
>>>> Maybe it is documented already and I missed it (which is perfectly
>>>> possible!).
>>> Well this is partly documented in
>>> Documentation/virtual/kvm/devices/arm-vgic-its.txt. No plan to
>>> deprecate. migration from KVM supporting v1 to KVM supporting V2 would
>>> be possible but not the contrary.
>>>
>>> Does it make sense?
>>
>> Sort of. Say you have three systems: A and C, which only supports v1; B,
>> which supports v1 and v2. Let's say you migrate from A to B, and from B
>> to C. Is B mandated to be able to export the tables as v1 and v2? Or can
>> it restrict what it can export?
> At the moment migration from B to C will fail because source ABI rev =
> v2 > destination support ABI = v1.
> 
> 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?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...