linux-kernel - Re: [PATCH v7 07/12] KVM: arm64: PMU: Set PMCR_EL0.N for vCPU based on the associated PMU

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Message-ID: <CAJHc60zQb0akx2Opbh3_Q8JShBC_9NFNvtAE+bPNi9QqXUGncA@mail.gmail.com>
Date:   Tue, 17 Oct 2023 09:58:08 -0700
From:   Raghavendra Rao Ananta <rananta@...gle.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     Sebastian Ott <sebott@...hat.com>, 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 07/12] KVM: arm64: PMU: Set PMCR_EL0.N for vCPU based
 on the associated PMU

On Mon, Oct 16, 2023 at 10:52 PM Oliver Upton <oliver.upton@...ux.dev> wrote:
>
> On Mon, Oct 16, 2023 at 02:35:52PM -0700, Raghavendra Rao Ananta wrote:
>
> [...]
>
> > > What's the point of doing this in the first place? The implementation of
> > > kvm_vcpu_read_pmcr() is populating PMCR_EL0.N using the VM-scoped value.
> > >
> > I guess originally the change replaced read_sysreg(pmcr_el0) with
> > kvm_vcpu_read_pmcr(vcpu) to maintain consistency with others.
> > But if you and Sebastian feel that it's an overkill and directly
> > getting the value via vcpu->kvm->arch.pmcr_n is more readable, I'm
> > happy to make the change.
>
> No, I'd rather you delete the line where PMCR_EL0.N altogether.
> reset_pmcr() tries to initialize the field, but your
> kvm_vcpu_read_pmcr() winds up replacing it with pmcr_n.
>
I didn't get this comment. We still do initialize pmcr, but using the
pmcr.n read via kvm_vcpu_read_pmcr() instead of the actual system
register.

Thank you.
Raghavendra
> > @@ -1105,8 +1109,16 @@ u8 kvm_arm_pmu_get_pmuver_limit(void)
> >  /**
> >   * kvm_vcpu_read_pmcr - Read PMCR_EL0 register for the vCPU
> >   * @vcpu: The vcpu pointer
> > + *
> > + * The function returns the value of PMCR.N based on the per-VM tracked
> > + * value (kvm->arch.pmcr_n). This is to ensure that the register field
> > + * remains consistent for the VM, even on heterogeneous systems where
> > + * the value may vary when read from different CPUs (during vCPU reset).
>
> Since I'm looking at this again, I don't believe the comment is adding
> much. KVM doesn't read pmcr_el0 directly anymore, and kvm_arch is
> clearly VM-scoped context.
>
> >   */
> >  u64 kvm_vcpu_read_pmcr(struct kvm_vcpu *vcpu)
> >  {
> > -     return __vcpu_sys_reg(vcpu, PMCR_EL0);
> > +     u64 pmcr = __vcpu_sys_reg(vcpu, PMCR_EL0) &
> > +                     ~(ARMV8_PMU_PMCR_N_MASK << ARMV8_PMU_PMCR_N_SHIFT);
> > +
> > +     return pmcr | ((u64)vcpu->kvm->arch.pmcr_n << ARMV8_PMU_PMCR_N_SHIFT);
> >  }
>
>
> --
> Thanks,
> Oliver