[PATCH v1 11/18] KVM: arm64: expose ID_AA64MMFR3_EL1 to guests

[Marc Zyngier]

On Thu, 09 Mar 2023 16:07:42 +0000,
Mark Brown <broonie at kernel.org> wrote:
> 
> [1  <text/plain; us-ascii (7bit)>]
> On Thu, Mar 09, 2023 at 02:52:39PM +0000, Joey Gouly wrote:
> 
> > Now that KVM context switches the appropriate registers, expose
> > ID_AA64MMFR3_EL1 to guests to allow them to use the new features.
> 
> Should we be adding new vCPU features given that there's new
> architectural state here?

What do we gain by that? AFAICT, it only makes the UAPI more complex.
And to be honest, *any* new feature added to KVM results in new
architectural state. Are we going to add more and more of these ad
nauseam? It doesn't scale. All we need to ensure at this stage is that
you cannot migrate a VM that has seen this to a host that doesn't have
the feature.

And if anything, this sort of selection should be defined by writing
to the ID_AA64MMFR3_EL1 register from userspace and let the whole
thing be driven by it. See Jing's current effort at [1].

Thanks,

	M.

[1] https://lore.kernel.org/r/20230228062246.1222387-1-jingzhangos@google.com

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