[PATCH v11 09/21] KVM: arm64: PMU: Recreate events after MDCR_EL2 changes

[Akihiko Odaki]

On 2026/09/22 15:54, Oliver Upton wrote:
> On Sun, Sep 20, 2026 at 08:15:50PM +0900, Akihiko Odaki wrote:
>> +void kvm_pmu_apply_mdcr(struct kvm_vcpu *vcpu, u64 old, u64 val)
>> +{
>> +	u64 changed = old ^ val;
>> +
>> +	/*
>> +	 * HPMN determines which counters HPMD and HLP apply to. Changes to
>> +	 * these fields require new perf event filters and sample periods.
>> +	 */
>> +	if (changed & (MDCR_EL2_HPMN | MDCR_EL2_HPMD | MDCR_EL2_HLP))
>> +		kvm_pmu_request_recreate(vcpu);
>> +	else if (changed & MDCR_EL2_HPME)
>> +		kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
>> +}
>> +
> 
> Urgh... I'm not a fan of this. Can you just modify kvm_vcpu_reload_pmu()
> to discard all backing perf events and recompute them only for enabled
> PMCs? We set KVM_REQ_RELOAD_PMU pretty rarely.
> 
>>   	__vcpu_rmw_sys_reg(vcpu, PMOVSSET_EL0, &=, mask);
>>   	__vcpu_rmw_sys_reg(vcpu, PMINTENSET_EL1, &=, mask);
>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index 75624725adf1..7dd2cafce247 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -3114,17 +3114,22 @@ static bool access_mdcr(struct kvm_vcpu *vcpu,
>>   	}
>>   
>>   	__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);
>> -
>> -	/*
>> -	 * Request a reload of the PMU to enable/disable the counters
>> -	 * affected by HPME.
>> -	 */
>> -	if ((old ^ val) & MDCR_EL2_HPME)
>> -		kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
>> +	kvm_pmu_apply_mdcr(vcpu, old, val);
>>   
>>   	return true;
>>   }
> 
>> +static int set_mdcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
>> +		    u64 val)
>> +{
>> +	u64 old = __vcpu_sys_reg(vcpu, MDCR_EL2);
>> +
>> +	__vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);
>> +	kvm_pmu_apply_mdcr(vcpu, old, val);
>> +
>> +	return 0;
>> +}
>> +
> 
> Just keep the old ^ new test inline for access_mdcr() and set the
> request unconditionally from set_mdcr(). In the usual case
> KVM_REQ_RELOAD_PMU is already set on a vCPU that userspace is restoring.

Hi Oliver,

Thanks for the feedback. I'll make both changes for v2.

I originally tried to preserve existing behavior and keep things 
consistent, but I agree that simplicity is much better here given how 
rarely KVM_REQ_RELOAD_PMU is set.

Regards,
Akihiko Odaki