linux-kernel - Re: [PATCH 06/17] KVM: arm64: Introduce method to partition the PMU

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Message-ID: <aD4llDZwb_sC_Ptj@linux.dev>
Date: Mon, 2 Jun 2025 15:28:36 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Colton Lewis <coltonlewis@...gle.com>
Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	Russell King <linux@...linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	Mark Rutland <mark.rutland@....com>, Shuah Khan <shuah@...nel.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-perf-users@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 06/17] KVM: arm64: Introduce method to partition the PMU

On Mon, Jun 02, 2025 at 07:26:51PM +0000, Colton Lewis wrote:
>  static void kvm_arm_setup_mdcr_el2(struct kvm_vcpu *vcpu)
>  {
> +	u8 hpmn = vcpu->kvm->arch.arm_pmu->hpmn;
> +
>  	preempt_disable();
>  
>  	/*
>  	 * This also clears MDCR_EL2_E2PB_MASK and MDCR_EL2_E2TB_MASK
>  	 * to disable guest access to the profiling and trace buffers
>  	 */
> -	vcpu->arch.mdcr_el2 = FIELD_PREP(MDCR_EL2_HPMN,
> -					 *host_data_ptr(nr_event_counters));
> -	vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
> +	vcpu->arch.mdcr_el2 = FIELD_PREP(MDCR_EL2_HPMN, hpmn);
> +	vcpu->arch.mdcr_el2 |= (MDCR_EL2_HPMD |
> +				MDCR_EL2_TPM |

This isn't safe, as there's no guarantee that kvm_arch::arm_pmu is
pointing that the PMU for this CPU. KVM needs to derive HPMN from some
per-CPU state, not anything tied to the VM/vCPU.

> +/**
> + * kvm_pmu_partition() - Partition the PMU
> + * @pmu: Pointer to pmu being partitioned
> + * @host_counters: Number of host counters to reserve
> + *
> + * Partition the given PMU by taking a number of host counters to
> + * reserve and, if it is a valid reservation, recording the
> + * corresponding HPMN value in the hpmn field of the PMU and clearing
> + * the guest-reserved counters from the counter mask.
> + *
> + * Passing 0 for @host_counters has the effect of disabling partitioning.
> + *
> + * Return: 0 on success, -ERROR otherwise
> + */
> +int kvm_pmu_partition(struct arm_pmu *pmu, u8 host_counters)
> +{
> +	u8 nr_counters;
> +	u8 hpmn;
> +
> +	if (!kvm_pmu_reservation_is_valid(host_counters))
> +		return -EINVAL;
> +
> +	nr_counters = *host_data_ptr(nr_event_counters);
> +	hpmn = kvm_pmu_hpmn(host_counters);
> +
> +	if (hpmn < nr_counters) {
> +		pmu->hpmn = hpmn;
> +		/* Inform host driver of available counters */
> +		bitmap_clear(pmu->cntr_mask, 0, hpmn);
> +		bitmap_set(pmu->cntr_mask, hpmn, nr_counters);
> +		clear_bit(ARMV8_PMU_CYCLE_IDX, pmu->cntr_mask);
> +		if (pmuv3_has_icntr())
> +			clear_bit(ARMV8_PMU_INSTR_IDX, pmu->cntr_mask);
> +
> +		kvm_debug("Partitioned PMU with HPMN %u", hpmn);
> +	} else {
> +		pmu->hpmn = nr_counters;
> +		bitmap_set(pmu->cntr_mask, 0, nr_counters);
> +		set_bit(ARMV8_PMU_CYCLE_IDX, pmu->cntr_mask);
> +		if (pmuv3_has_icntr())
> +			set_bit(ARMV8_PMU_INSTR_IDX, pmu->cntr_mask);
> +
> +		kvm_debug("Unpartitioned PMU");
> +	}
> +
> +	return 0;
> +}

Hmm... Just in terms of code organization I'm not sure I like having KVM
twiddling with *host* support for PMUv3. Feels like the ARM PMU driver
should own partitioning and KVM just takes what it can get.

> @@ -239,6 +245,13 @@ void kvm_host_pmu_init(struct arm_pmu *pmu)
>  	if (!pmuv3_implemented(kvm_arm_pmu_get_pmuver_limit()))
>  		return;
>  
> +	if (reserved_host_counters) {
> +		if (kvm_pmu_partition_supported())
> +			WARN_ON(kvm_pmu_partition(pmu, reserved_host_counters));
> +		else
> +			kvm_err("PMU Partition is not supported");
> +	}
> +

Hasn't the ARM PMU been registered with perf at this point? Surely the
driver wouldn't be very pleased with us ripping counters out from under
its feet.

Thanks,
Oliver