linux-kernel - Re: [PATCH v7 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR

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Message-ID: <5d35c9f3-455e-6aa9-fd6a-4433cf70803a@redhat.com>
Date:   Tue, 17 Oct 2023 17:52:37 +0200 (CEST)
From:   Sebastian Ott <sebott@...hat.com>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
cc:     Oliver Upton <oliver.upton@...ux.dev>,
        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 v7 08/12] KVM: arm64: PMU: Allow userspace to limit
 PMCR_EL0.N for the guest

On Mon, 9 Oct 2023, Raghavendra Rao Ananta wrote:
> +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;
> +
> +	mutex_lock(&kvm->arch.config_lock);
> +
> +	/*
> +	 * Make PMCR immutable once the VM has started running, but do
> +	 * not return an error (-EBUSY) to meet the existing expectations.
> +	 */

Why should we mention which error we're _not_ returning?


> +	if (kvm_vm_has_ran_once(vcpu->kvm)) {
> +		mutex_unlock(&kvm->arch.config_lock);
> +		return 0;
> +	}