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:   Tue, 17 Oct 2023 09:59:39 -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

Hi Eric,
On Tue, Oct 17, 2023 at 2:23 AM Eric Auger <eauger@...hat.com> wrote:
>
> Hi,
> On 10/16/23 23:28, Raghavendra Rao Ananta wrote:
> > 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?
> Then why do we bother using reset_unknown in the other places if
> clearing the bits is enough here?
>
Hmm. Good point. I can bring back reset_unknown to keep the original behavior.

Thank you.
Raghavendra
> Thanks
>
> Eric
> >
> > 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 */
> >>
> >
>