linux-kernel - Re: [PATCH v8 06/13] KVM: arm64: Sanitize PM{C,I}NTEN{SET,CLR}, PMOVS{SET,CLR} before first run

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Message-ID: <86ttqh43hq.wl-maz@kernel.org>
Date:   Mon, 23 Oct 2023 19:07:13 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     Oliver Upton <oliver.upton@...ux.dev>,
        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 06/13] KVM: arm64: Sanitize PM{C,I}NTEN{SET,CLR}, PMOVS{SET,CLR} before first run

On Mon, 23 Oct 2023 18:42:43 +0100,
Raghavendra Rao Ananta <rananta@...gle.com> wrote:
> 
> On Mon, Oct 23, 2023 at 5:42 AM Marc Zyngier <maz@...nel.org> wrote:
> >
> > On Fri, 20 Oct 2023 22:40:46 +0100,
> > Raghavendra Rao Ananta <rananta@...gle.com> wrote:
> > >
> > > For unimplemented counters, the registers PM{C,I}NTEN{SET,CLR}
> > > and PMOVS{SET,CLR} are expected to have the corresponding bits RAZ.
> > > Hence to ensure correct KVM's PMU emulation, mask out the bits in
> > > these registers for these unimplemented counters before the first
> > > vCPU run.
> > >
> > > Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
> > > ---
> > >  arch/arm64/kvm/arm.c      |  2 +-
> > >  arch/arm64/kvm/pmu-emul.c | 11 +++++++++++
> > >  include/kvm/arm_pmu.h     |  2 ++
> > >  3 files changed, 14 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > > index e3074a9e23a8b..3c0bb80483fb1 100644
> > > --- a/arch/arm64/kvm/arm.c
> > > +++ b/arch/arm64/kvm/arm.c
> > > @@ -857,7 +857,7 @@ static int check_vcpu_requests(struct kvm_vcpu *vcpu)
> > >               }
> > >
> > >               if (kvm_check_request(KVM_REQ_RELOAD_PMU, vcpu))
> > > -                     kvm_pmu_handle_pmcr(vcpu, kvm_vcpu_read_pmcr(vcpu));
> > > +                     kvm_vcpu_handle_request_reload_pmu(vcpu);
> >
> > Please rename this to kvm_vcpu_reload_pmu(). That's long enough. But
> > see below.
> >
> Sounds good.
> 
> > >
> > >               if (kvm_check_request(KVM_REQ_RESYNC_PMU_EL0, vcpu))
> > >                       kvm_vcpu_pmu_restore_guest(vcpu);
> > > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> > > index 9e24581206c24..31e4933293b76 100644
> > > --- a/arch/arm64/kvm/pmu-emul.c
> > > +++ b/arch/arm64/kvm/pmu-emul.c
> > > @@ -788,6 +788,17 @@ u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu, bool pmceid1)
> > >       return val & mask;
> > >  }
> > >
> > > +void kvm_vcpu_handle_request_reload_pmu(struct kvm_vcpu *vcpu)
> > > +{
> > > +     u64 mask = kvm_pmu_valid_counter_mask(vcpu);
> > > +
> > > +     kvm_pmu_handle_pmcr(vcpu, kvm_vcpu_read_pmcr(vcpu));
> > > +
> > > +     __vcpu_sys_reg(vcpu, PMOVSSET_EL0) &= mask;
> > > +     __vcpu_sys_reg(vcpu, PMINTENSET_EL1) &= mask;
> > > +     __vcpu_sys_reg(vcpu, PMCNTENSET_EL0) &= mask;
> > > +}
> >
> > Why is this done on a vcpu request? Why can't it be done upfront, when
> > we're requesting the reload? Or when assigning the PMU? Or when
> > setting PMCR_EL0?
> >
> The idea was to do this only once, after userspace has configured the
> PMCR.N (and has no option to change it), but before we run the guest
> for the first time. So, I guess this can be done when we are
> requesting the reload, if you prefer.

Well, I'm trying to limit the proliferation of these one-off "helpers"
that make the code hard to follow. So it isn't "what I prefer", but
what makes the code easier to understand without having to follow a
maze of pointless abstraction.

> When assigning the PMU, it could be too early to sanitize as the
> userspace would not have configured the PMCR.N yet.
> It can probably be done when userspace configures PMCR.N, but since
> this field is per-guest, we may have to apply the setting for all the
> vCPUs during the ioctl, which may get a little ugly.

Right. So it has to happen at the point where userspace cannot write
to PMCR_EL0 anymore, for which any of the options I mentioned is too
early. What you have is thus correct. But it would have helped if that
rationale was captured in the commit message.

	M.

-- 
Without deviation from the norm, progress is not possible.