linux-kernel - Re: [PATCH v8 07/13] KVM: arm64: PMU: Allow userspace to limit PMCR

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Date:   Tue, 24 Oct 2023 18:37:17 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>,
        Alexandru Elisei <alexandru.elisei@....com>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Shaoqin Huang <shahuang@...hat.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Reiji Watanabe <reijiw@...gle.com>,
        Colton Lewis <coltonlewis@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v8 07/13] KVM: arm64: PMU: Allow userspace to limit
 PMCR_EL0.N for the guest

On Fri, Oct 20, 2023 at 09:40:47PM +0000, Raghavendra Rao Ananta wrote:

[...]

> +	/*
> +	 * Make PMCR immutable once the VM has started running, but
> +	 * do not return an error to meet the existing expectations.
> +	 */
> +	if (kvm_vm_has_ran_once(vcpu->kvm)) {
> +		mutex_unlock(&kvm->arch.config_lock);
> +		return 0;
> +	}

Marc pointed out offline that PMCR_EL0 needs to be mutable at runtime as
well. I'll admit, the architecture isn't very helpful as it is both used
for identification _and_ configuration.

What I had in mind a few revisions ago when I gave the suggestion was to
ignore writes to _just_ the PMCR_EL0.N field after the VM has started.

-- 
Thanks,
Oliver