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

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Message-ID: <CAJHc60z7U1-irTy-6URb_V0PTW+TYS4qodf2akSg33_7CJgjyw@mail.gmail.com>
Date:   Tue, 17 Oct 2023 09:49:07 -0700
From:   Raghavendra Rao Ananta <rananta@...gle.com>
To:     Sebastian Ott <sebott@...hat.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 Tue, Oct 17, 2023 at 8:52 AM Sebastian Ott <sebott@...hat.com> wrote:
>
> 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?
>
Oh, it's not to break the existing userspace expectations. Before this
series, any 'write' from userspace was possible. Returning -EBUSY all
of a sudden might tamper with this expectation.

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