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

[Marc Zyngier]

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.

> 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.

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.

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.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.