Re: [PATCH v5 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest

[Oliver Upton]

Hi Raghu,

On Thu, Aug 17, 2023 at 12:30:25AM +0000, Raghavendra Rao Ananta wrote:
> From: Reiji Watanabe <reijiw@xxxxxxxxxx>
> 
> KVM does not yet support userspace modifying PMCR_EL0.N (With
> the previous patch, KVM ignores what is written by upserspace).

typo: userspace

> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index ce7de6bbdc967..39ad56a71ad20 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -896,6 +896,7 @@ int kvm_arm_set_vm_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
>  	 * while the latter does not.
>  	 */
>  	kvm->arch.pmcr_n = arm_pmu->num_events - 1;
> +	kvm->arch.pmcr_n_limit = arm_pmu->num_events - 1;

Can't we just get at this through the arm_pmu instance rather than
copying it into kvm_arch?

>  	return 0;
>  }
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 2075901356c5b..c01d62afa7db4 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1086,6 +1086,51 @@ static int get_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
>  	return 0;
>  }
>  
> +static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
> +		    u64 val)
> +{
> +	struct kvm *kvm = vcpu->kvm;
> +	u64 new_n, mutable_mask;
> +	int ret = 0;
> +
> +	new_n = FIELD_GET(ARMV8_PMU_PMCR_N, val);
> +
> +	mutex_lock(&kvm->arch.config_lock);
> +	if (unlikely(new_n != kvm->arch.pmcr_n)) {
> +		/*
> +		 * The vCPU can't have more counters than the PMU
> +		 * hardware implements.
> +		 */
> +		if (new_n <= kvm->arch.pmcr_n_limit)
> +			kvm->arch.pmcr_n = new_n;
> +		else
> +			ret = -EINVAL;
> +	}

Hmm, I'm not so sure about returning an error here. ABI has it that
userspace can write any value to PMCR_EL0 successfully. Can we just
ignore writes that attempt to set PMCR_EL0.N to something higher than
supported by hardware? Our general stance should be that system register
fields responsible for feature identification are immutable after the VM
has started.

-- 
Thanks,
Oliver