Re: [PATCH] KVM: arm64: Don't let a guest overwrite HCRX_EL2.MCE2

[Marc Zyngier <maz@kernel.org>]

On Wed, 28 Oct 2026 00:12:00 +0000,
Oliver Upton <oupton@kernel.org> wrote:
> 
> Hey,
> 
> On Mon, Oct 26, 2026 at 08:26:33AM +0000, Marc Zyngier wrote:
> > FEAT_MOPS is fun. CPUs are allowed to implement any of two ways to
> > perform the memory copy, and the hypervisor is in charge of hiding
> > the difference between implementation when the vcpu migrates from
> > one to another. This is unhelpfully signalled with an exception,
> > which KVM diligently handles.
> > 
> > Today, we don't really make a good job of it in the sense that
> > if L1 also has the trap bit set, we don't forward the trap as
> > the architecture would require us to do.
> > 
> > But there is no good reason to forward this to the L1 guest if
> > L2 generates the "I'm on the wrong implementation". KVM can fix
> > it up in place and return to L2 directly, as if the physical CPU
> > was able to implement both options.
> > 
> > Conversely, allowing L1 to clear HCRX_EL2.MCE2 is counter-productive,
> > and violates the abstraction that KVM aims to give.
> 
> What abstraction? :-)
> 
> At least in the non-nested case we can blame the VMM since it's
> responsible for scheduling vCPUs on the right pCPUs in a heterogeneous
> system.

The problem is that there is no ID register describing which
architectural option is implemented on a given CPU.

> There is a subtlety here where if the VMM does the right thing with
> sched affinities then a MOPS exception taken from L2 is definitely the
> L1's fault and architecturally visible.

How? It isn't discoverable the first place. And migration breaks this
entirely. The whole A/B thing is an absolute disaster.

> 
> But oh well. We're obviously oriented towards presenting a homogeneous
> set of vCPUs and anything else works by accident.
> 
> > Implement this behaviour by never allowing the guest's view of
> > HCRX_EL2.MCE2 to make it into the HW, and document that KVM is in
> > charge of the A/B handling.
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> 
> Reviewed-by: Oliver Upton <oupton@kernel.org>

Thanks!

	M.

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