[PATCH] KVM: arm64: Disabling disabled PMU counters wastes a lot of time
Marc Zyngier
maz at kernel.org
Tue Jun 29 07:25:13 PDT 2021
On 2021-06-29 15:17, Alexandre Chartre wrote:
> On 6/29/21 3:47 PM, Marc Zyngier wrote:
>> On Tue, 29 Jun 2021 14:16:55 +0100,
>> Alexandre Chartre <alexandre.chartre at oracle.com> wrote:
>>>
>>>
>>> Hi Marc,
>>>
>>> On 6/29/21 11:06 AM, Marc Zyngier wrote:
>>>> Hi Alexandre,
>>
>> [...]
>>
>>>> So the sysreg is the only thing we should consider, and I think we
>>>> should drop the useless masking. There is at least another instance
>>>> of
>>>> this in the PMU code (kvm_pmu_overflow_status()), and apart from
>>>> kvm_pmu_vcpu_reset(), only the sysreg accessors should care about
>>>> the
>>>> masking to sanitise accesses.
>>>>
>>>> What do you think?
>>>>
>>>
>>> I think you are right. PMCNTENSET_EL0 is already masked with
>>> kvm_pmu_valid_counter_mask() so there's effectively no need to mask
>>> it again when we use it. I will send an additional patch (on top of
>>> this one) to remove useless masking. Basically, changes would be:
>>>
>>> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
>>> index bab4b735a0cf..e0dfd7ce4ba0 100644
>>> --- a/arch/arm64/kvm/pmu-emul.c
>>> +++ b/arch/arm64/kvm/pmu-emul.c
>>> @@ -373,7 +373,6 @@ static u64 kvm_pmu_overflow_status(struct
>>> kvm_vcpu *vcpu)
>>> reg = __vcpu_sys_reg(vcpu, PMOVSSET_EL0);
>>> reg &= __vcpu_sys_reg(vcpu, PMCNTENSET_EL0);
>>> reg &= __vcpu_sys_reg(vcpu, PMINTENSET_EL1);
>>> - reg &= kvm_pmu_valid_counter_mask(vcpu);
>>> }
>>> return reg;
>>> @@ -564,21 +563,22 @@ void kvm_pmu_software_increment(struct kvm_vcpu
>>> *vcpu, u64 val)
>>> */
>>> void kvm_pmu_handle_pmcr(struct kvm_vcpu *vcpu, u64 val)
>>> {
>>> - unsigned long mask = kvm_pmu_valid_counter_mask(vcpu);
>>> + unsigned long mask;
>>> int i;
>>> if (val & ARMV8_PMU_PMCR_E) {
>>> kvm_pmu_enable_counter_mask(vcpu,
>>> - __vcpu_sys_reg(vcpu, PMCNTENSET_EL0) & mask);
>>> + __vcpu_sys_reg(vcpu, PMCNTENSET_EL0));
>>> } else {
>>> kvm_pmu_disable_counter_mask(vcpu,
>>> - __vcpu_sys_reg(vcpu, PMCNTENSET_EL0) & mask);
>>> + __vcpu_sys_reg(vcpu, PMCNTENSET_EL0));
>>> }
>>> if (val & ARMV8_PMU_PMCR_C)
>>> kvm_pmu_set_counter_value(vcpu, ARMV8_PMU_CYCLE_IDX,
>>> 0);
>>> if (val & ARMV8_PMU_PMCR_P) {
>>> + mask = kvm_pmu_valid_counter_mask(vcpu);
>>
>> Careful here, this clashes with a fix from Alexandru that is currently
>> in -next (PMCR_EL0.P shouldn't reset the cycle counter) and aimed at
>> 5.14. And whilst you're at it, consider moving the 'mask' declaration
>> here too.
>>
>>> for_each_set_bit(i, &mask, 32)
>>> kvm_pmu_set_counter_value(vcpu, i, 0);
>>> }
>>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>>> index 1a7968ad078c..2e406905760e 100644
>>> --- a/arch/arm64/kvm/sys_regs.c
>>> +++ b/arch/arm64/kvm/sys_regs.c
>>> @@ -845,7 +845,7 @@ static bool access_pmcnten(struct kvm_vcpu *vcpu,
>>> struct sys_reg_params *p,
>>> kvm_pmu_disable_counter_mask(vcpu, val);
>>> }
>>> } else {
>>> - p->regval = __vcpu_sys_reg(vcpu, PMCNTENSET_EL0) &
>>> mask;
>>> + p->regval = __vcpu_sys_reg(vcpu, PMCNTENSET_EL0);
>>> }
>>> return true;
>>
>> If you are cleaning up the read-side of sysregs, access_pminten() and
>> access_pmovs() could have some of your attention too.
>>
>
> Ok, so for now, I will just resubmit the initial patch with the commit
> comment fixes. Then, look at all the mask cleanup on top of Alexandru
> changes and prepare another patch.
Please send this as a series rather than individual patches. I'm only
queuing critical fixes at the moment (this is the merge window).
If you post the series after -rc1, I'll queue it and let it simmer
in -next.
Thanks,
M.
--
Jazz is not dead. It just smells funny...
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