linux-kernel - Re: [PATCH v7 03/12] KVM: arm64: PMU: Clear PM{C,I}NTEN{SET,CLR} and PMOVS{SET,CLR} on vCPU reset

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Date:   Mon, 16 Oct 2023 14:28:58 -0700
From:   Raghavendra Rao Ananta <rananta@...gle.com>
To:     Eric Auger <eauger@...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 03/12] KVM: arm64: PMU: Clear PM{C,I}NTEN{SET,CLR} and
 PMOVS{SET,CLR} on vCPU reset

On Mon, Oct 16, 2023 at 12:45 PM Eric Auger <eauger@...hat.com> wrote:
>
> Hi Raghavendra,
>
> On 10/10/23 01:08, Raghavendra Rao Ananta wrote:
> > From: Reiji Watanabe <reijiw@...gle.com>
> >
> > On vCPU reset, PMCNTEN{SET,CLR}_EL0, PMINTEN{SET,CLR}_EL1, and
> > PMOVS{SET,CLR}_EL1 for a vCPU are reset by reset_pmu_reg().
> PMOVS{SET,CLR}_EL0?
Ah, yes. It should be PMOVS{SET,CLR}_EL0.

> > This function clears RAZ bits of those registers corresponding
> > to unimplemented event counters on the vCPU, and sets bits
> > corresponding to implemented event counters to a predefined
> > pseudo UNKNOWN value (some bits are set to 1).
> >
> > The function identifies (un)implemented event counters on the
> > vCPU based on the PMCR_EL0.N value on the host. Using the host
> > value for this would be problematic when KVM supports letting
> > userspace set PMCR_EL0.N to a value different from the host value
> > (some of the RAZ bits of those registers could end up being set to 1).
> >
> > Fix this by clearing the registers so that it can ensure
> > that all the RAZ bits are cleared even when the PMCR_EL0.N value
> > for the vCPU is different from the host value. Use reset_val() to
> > do this instead of fixing reset_pmu_reg(), and remove
> > reset_pmu_reg(), as it is no longer used.
> do you intend to restore the 'unknown' behavior at some point?
>
I believe Reiji's (original author) intention was to keep them
cleared, which would still imply an 'unknown' behavior. Do you think
there's an issue with this?

Thank you.
Raghavendra
> Thanks
>
> Eric
> >
> > Signed-off-by: Reiji Watanabe <reijiw@...gle.com>
> > Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
> > ---
> >  arch/arm64/kvm/sys_regs.c | 21 +--------------------
> >  1 file changed, 1 insertion(+), 20 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 818a52e257ed..3dbb7d276b0e 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -717,25 +717,6 @@ static unsigned int pmu_visibility(const struct kvm_vcpu *vcpu,
> >       return REG_HIDDEN;
> >  }
> >
> > -static u64 reset_pmu_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
> > -{
> > -     u64 n, mask = BIT(ARMV8_PMU_CYCLE_IDX);
> > -
> > -     /* No PMU available, any PMU reg may UNDEF... */
> > -     if (!kvm_arm_support_pmu_v3())
> > -             return 0;
> > -
> > -     n = read_sysreg(pmcr_el0) >> ARMV8_PMU_PMCR_N_SHIFT;
> > -     n &= ARMV8_PMU_PMCR_N_MASK;
> > -     if (n)
> > -             mask |= GENMASK(n - 1, 0);
> > -
> > -     reset_unknown(vcpu, r);
> > -     __vcpu_sys_reg(vcpu, r->reg) &= mask;
> > -
> > -     return __vcpu_sys_reg(vcpu, r->reg);
> > -}
> > -
> >  static u64 reset_pmevcntr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
> >  {
> >       reset_unknown(vcpu, r);
> > @@ -1115,7 +1096,7 @@ static bool access_pmuserenr(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
> >         trap_wcr, reset_wcr, 0, 0,  get_wcr, set_wcr }
> >
> >  #define PMU_SYS_REG(name)                                            \
> > -     SYS_DESC(SYS_##name), .reset = reset_pmu_reg,                   \
> > +     SYS_DESC(SYS_##name), .reset = reset_val,                       \
> >       .visibility = pmu_visibility
> >
> >  /* Macro to expand the PMEVCNTRn_EL0 register */
>