[PATCH] KVM: arm64: Make the exposed feature bits in AA64DFR0_EL1 writable from userspace

[Eric Auger]

Hi Marc,

On 12/1/24 13:21, Marc Zyngier wrote:
> Hey Eric,
> 
> On Thu, 28 Nov 2024 09:31:08 +0000,
> Eric Auger <eauger at redhat.com> wrote:
>>
>> Hi Marc,
>>
>> On 11/26/24 20:29, Marc Zyngier wrote:
>>> Finally, who is going to ensure this keeps working in the foreseeable
>>> future? Because while this is nice, that's not what gets deployed in
>>> production, as it leads to unpredictable performances. My take is that
>>> this thing will eventually bitrot and die.
>> In the context of our works to define qemu vcpu models for ARM
>> (https://lore.kernel.org/all/20241025101959.601048-1-eric.auger@redhat.com/)
>> , our current approach is to try migrating between modern HW we have
>> access to. The case above is migration between AmpereOne and Grace which
>> both should be prevalent systems. Do you think this does not make sense
>> at all to try migrating between those, alhough this may be challenging?
> 
> I don't mind the challenge. But I'm worried this is something that
> looks like a reasonable idea that doesn't get any traction in
> practice.
> 
> And the example you mention is pretty striking: who in their right
> mind would migrate between these two systems? If you deploy a Grace
> system, that's because you are making use of the GPU, and your VM is
> likely to require it. Conversely, if you run on an Ampere system, you
> don't want to use a valuable (read: bloody expensive) slot on a Grace
> machine.

Yes I acknowledge it is a total valid point from a use case and cost
point of view. I was expecting maybe some interest migrating between
AmpereOne and Grace-Grace for farm enhancement but most probably it is
marginal.

Definition of [qemu] named vcpu models looks pretty uneasy then because
we don't have much relevant and accessible HW to test with, taking into
account such non technical considerations. Besides migration within a
CPU family I don't see much.
> 
>> Other cases we have looked at are migration within Ampere Altra Max
>> system family (which should be hopefully fine now with have CTR_EL0
>> works from Sebastian upstream), mig between Graviton hosts. Wrt Ampere
>> Altra Max to AmpereOne, Oliver pointed out the cntfrq issue which is
>> blocking.
>>
>> Do you think we should restrict our studies to systems which are
>> "closer" to each other in terms of ARM spec rev. We throught that
>> migration bewteen AmpereOne And Grace would be an interesting POC and
>> not totally irrelevant in terms of industry.
> 
> These two implementations may be close in terms of CPU features. But
> as systems, they are massively different, and I very much doubt they
> have the same deployment story. If they have one at all.
OK thank you for sharing your point of view.
> 
> The Graviton story may have more traction, but these folks have their
> own way of doing things, and in my experience do not give upstream
> much consideration.
OK thanks
> 
> To sum it up, I'm not opposed to this work. But if we are going to
> carry this sort of complex emulation, I want someone to step up and
> promise that they will test it for the next 10 years, at the very
> least. Because I'm very unlikely to ever have access to any of these
> machines, let alone both, and I don't see people using it in practice.
understood!

Thanks

Eric
> 
> Thanks,
> 
> 	M.
>