Re: [PATCH v7 07/12] KVM: arm64: PMU: Set PMCR_EL0.N for vCPU based on the associated PMU

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

On Mon, Oct 16, 2023 at 02:35:52PM -0700, Raghavendra Rao Ananta wrote:

[...]

> > What's the point of doing this in the first place? The implementation of
> > kvm_vcpu_read_pmcr() is populating PMCR_EL0.N using the VM-scoped value.
> >
> I guess originally the change replaced read_sysreg(pmcr_el0) with
> kvm_vcpu_read_pmcr(vcpu) to maintain consistency with others.
> But if you and Sebastian feel that it's an overkill and directly
> getting the value via vcpu->kvm->arch.pmcr_n is more readable, I'm
> happy to make the change.

No, I'd rather you delete the line where PMCR_EL0.N altogether.
reset_pmcr() tries to initialize the field, but your
kvm_vcpu_read_pmcr() winds up replacing it with pmcr_n.

> @@ -1105,8 +1109,16 @@ u8 kvm_arm_pmu_get_pmuver_limit(void)
>  /**
>   * kvm_vcpu_read_pmcr - Read PMCR_EL0 register for the vCPU
>   * @vcpu: The vcpu pointer
> + *
> + * The function returns the value of PMCR.N based on the per-VM tracked
> + * value (kvm->arch.pmcr_n). This is to ensure that the register field
> + * remains consistent for the VM, even on heterogeneous systems where
> + * the value may vary when read from different CPUs (during vCPU reset).

Since I'm looking at this again, I don't believe the comment is adding
much. KVM doesn't read pmcr_el0 directly anymore, and kvm_arch is
clearly VM-scoped context.

>   */
>  u64 kvm_vcpu_read_pmcr(struct kvm_vcpu *vcpu)
>  {
> -	return __vcpu_sys_reg(vcpu, PMCR_EL0);
> +	u64 pmcr = __vcpu_sys_reg(vcpu, PMCR_EL0) &
> +			~(ARMV8_PMU_PMCR_N_MASK << ARMV8_PMU_PMCR_N_SHIFT);
> +
> +	return pmcr | ((u64)vcpu->kvm->arch.pmcr_n << ARMV8_PMU_PMCR_N_SHIFT);
>  }


-- 
Thanks,
Oliver