Re: [PATCH v2 2/2] KVM: arm64: Skip unreset vCPUs in MPIDR lookup table

[Marc Zyngier <maz@kernel.org>]

On Thu, 18 Jun 2026 12:38:54 +0100,
fuqiang wang <fuqiang.wng@gmail.com> wrote:
> 
> However, could we detect collisions within the init function and, upon
> detection, notify userspace without taking any corrective or fallback
> action? This would serve as a stronger reminder to userspace of its
> non-compliant behavior.
> 
> e.g.
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 3732ee9eb0d4..7563feab1a11 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -893,6 +893,17 @@ static void kvm_init_mpidr_data(struct kvm *kvm)
>                 data->cmpidr_to_idx[index] = c;
>         }
> 
> +       kvm_for_each_vcpu(c, vcpu, kvm) {
> +               u64 aff = kvm_vcpu_get_mpidr_aff(vcpu);
> +               u16 index = kvm_mpidr_index(data, aff);
> +
> +               if (data->cmpidr_to_idx[index] != c) {
> +                       pr_warn("Multiple vCPUs share the same MPIDR value, "
> +                               "it may cause the guest to hang or run slower\n");
> +                       break;
> +               }
> +       }
> +

That's not reporting anything to userspace. That's just screaming in
the kernel log, and a random user does not have access to that.

If you wanted the user to take notice, you'd need to fail the entry
into the guest.

	M.

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