[PATCH] KVM: arm64: Disabling disabled PMU counters wastes a lot of time

[Alexandre Chartre]

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.

alex.