Re: [PATCH v6 02/11] KVM: arm64: PMU: Set the default PMU for the guest on vCPU reset

From: Oliver Upton <oliver.upton@linux.dev>
To: Raghavendra Rao Ananta <rananta@google.com>
Cc: Marc Zyngier <maz@kernel.org>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Shaoqin Huang <shahuang@redhat.com>,
	Jing Zhang <jingzhangos@google.com>,
	Reiji Watanabe <reijiw@google.com>,
	Colton Lewis <coltonlewis@google.com>,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v6 02/11] KVM: arm64: PMU: Set the default PMU for the guest on vCPU reset
Date: Wed, 27 Sep 2023 08:02:40 +0000	[thread overview]
Message-ID: <ZRPhoExoiU3_Jvxy@linux.dev> (raw)
In-Reply-To: <20230926234008.2348607-3-rananta@google.com>

Hi Raghu,

On Tue, Sep 26, 2023 at 11:39:59PM +0000, Raghavendra Rao Ananta wrote:
> From: Reiji Watanabe <reijiw@google.com>
> 
> The following patches will use the number of counters information
> from the arm_pmu and use this to set the PMCR.N for the guest
> during vCPU reset. However, since the guest is not associated
> with any arm_pmu until userspace configures the vPMU device
> attributes, and a reset can happen before this event, call
> kvm_arm_support_pmu_v3() just before doing the reset.
> 
> No functional change intended.

I would argue there still is a functional change here, as PMU
initialization failure now shows up on a completely different ioctl for
userspace.

> @@ -216,6 +217,18 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
>  	vcpu->arch.reset_state.reset = false;
>  	spin_unlock(&vcpu->arch.mp_state_lock);
>  
> +	if (kvm_vcpu_has_pmu(vcpu)) {
> +		if (!kvm_arm_support_pmu_v3())
> +			return -EINVAL;
> +
> +		/*
> +		 * When the vCPU has a PMU, but no PMU is set for the guest
> +		 * yet, set the default one.
> +		 */
> +		if (unlikely(!kvm->arch.arm_pmu) && kvm_arm_set_default_pmu(kvm))
> +			return -EINVAL;
> +	}
> +

Ah, this probably will not mix well with my recent change to get rid of
the return value altogether from kvm_reset_vcpu() [*]. I see two ways to
handle this:

 - Add a separate helper responsible for one-time setup of the vCPU
   called from KVM_ARM_VCPU_INIT which may fail.

 - Add a check for !kvm->arch.arm_pmu to kvm_arm_pmu_v3_init().

No strong preference, though.

[*]: https://lore.kernel.org/r/20230920195036.1169791-8-oliver.upton@linux.dev

-- 
Thanks,
Oliver