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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Message-ID: <34959db4-01e9-8c1e-110e-c52701e2fb19@redhat.com>
Date:   Tue, 17 Oct 2023 11:23:42 +0200
From:   Eric Auger <eauger@...hat.com>
To:     Raghavendra Rao Ananta <rananta@...gle.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,
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?

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 */
>>
>