Re: [PATCH v2 1/6] KVM: arm64: Fix MDCR_EL2.HPMN reset value

[Oliver Upton <oliver.upton@linux.dev>]

On Wed, Apr 09, 2025 at 05:01:01PM +0100, Marc Zyngier wrote:
> The MDCR_EL2 documentation indicates that the HPMN field has
> the following behaviour:
> 
> "On a Warm reset, this field resets to the expression NUM_PMU_COUNTERS."
> 
> However, it appears we reset it to zero, which is not very useful.
> 
> Add a reset helper for MDCR_EL2, and handle the case where userspace
> changes the target PMU, which may force us to change HPMN again.
> 
> Reported-by: Joey Gouly <joey.gouly@arm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/pmu-emul.c | 13 +++++++++++++
>  arch/arm64/kvm/sys_regs.c |  8 +++++++-
>  2 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index a1bc10d7116a5..4dc4f3a473c3f 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -1033,6 +1033,19 @@ static void kvm_arm_set_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
>  
>  	kvm->arch.arm_pmu = arm_pmu;
>  	kvm->arch.pmcr_n = kvm_arm_pmu_get_max_counters(kvm);

nit: Can we rename pmcr_n to nr_pmu_counters? The current name is misleading.

> +
> +	/* Reset MDCR_EL2.HPMN behind the vcpus' back... */
> +	if (test_bit(KVM_ARM_VCPU_HAS_EL2, kvm->arch.vcpu_features)) {
> +		struct kvm_vcpu *vcpu;
> +		unsigned long i;
> +
> +		kvm_for_each_vcpu(i, vcpu, kvm) {
> +			u64 val = __vcpu_sys_reg(vcpu, MDCR_EL2);
> +			val &= ~MDCR_EL2_HPMN;
> +			val |= FIELD_PREP(MDCR_EL2_HPMN, kvm->arch.pmcr_n);
> +			__vcpu_sys_reg(vcpu, MDCR_EL2) = val;
> +		}

Shouldn't we be taking the vCPU mutex(es) here?

Thanks,
Oliver