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: <ZS6_tdkS6GyNlt4l@linux.dev>
Date:   Tue, 17 Oct 2023 17:09:09 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
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 Tue, Oct 17, 2023 at 09:58:08AM -0700, Raghavendra Rao Ananta wrote:
> 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.

You have two bits of code trying to do the exact same thing:

 1) reset_pmcr() initializes __vcpu_sys_reg(vcpu, PMCR_EL0) with the N
    field set up.

 2) kvm_vcpu_read_pmcr() takes whatever is in __vcpu_sys_reg(vcpu, PMCR_EL0),
    *masks out* the N field and re-initializes it with vcpu->kvm->arch.pmcr_n

Why do you need (1) if you do (2)?

-- 
Thanks,
Oliver