linux-kernel - Re: [PATCH v5 16/24] KVM: arm64: Account for partitioning in PMCR

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Message-ID: <gsntms3n2xq5.fsf@coltonlewis-kvm.c.googlers.com>
Date: Fri, 12 Dec 2025 21:31:46 +0000
From: Colton Lewis <coltonlewis@...gle.com>
To: Oliver Upton <oupton@...nel.org>
Cc: kvm@...r.kernel.org, pbonzini@...hat.com, corbet@....net, 
	linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org, 
	maz@...nel.org, oliver.upton@...ux.dev, mizhang@...gle.com, 
	joey.gouly@....com, suzuki.poulose@....com, yuzenghui@...wei.com, 
	mark.rutland@....com, shuah@...nel.org, gankulkarni@...amperecomputing.com, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	linux-perf-users@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v5 16/24] KVM: arm64: Account for partitioning in PMCR_EL0 access

Oliver Upton <oupton@...nel.org> writes:

> On Tue, Dec 09, 2025 at 08:51:13PM +0000, Colton Lewis wrote:
>> Make sure reads and writes to PMCR_EL0 conform to additional
>> constraints imposed when the PMU is partitioned.

>> Signed-off-by: Colton Lewis <coltonlewis@...gle.com>
>> ---
>>   arch/arm64/kvm/pmu.c      | 2 +-
>>   arch/arm64/kvm/sys_regs.c | 2 +-
>>   2 files changed, 2 insertions(+), 2 deletions(-)

>> diff --git a/arch/arm64/kvm/pmu.c b/arch/arm64/kvm/pmu.c
>> index 1fd012f8ff4a9..48b39f096fa12 100644
>> --- a/arch/arm64/kvm/pmu.c
>> +++ b/arch/arm64/kvm/pmu.c
>> @@ -877,7 +877,7 @@ u64 kvm_pmu_accessible_counter_mask(struct kvm_vcpu  
>> *vcpu)
>>   u64 kvm_vcpu_read_pmcr(struct kvm_vcpu *vcpu)
>>   {
>>   	u64 pmcr = __vcpu_sys_reg(vcpu, PMCR_EL0);
>> -	u64 n = vcpu->kvm->arch.nr_pmu_counters;
>> +	u64 n = kvm_pmu_guest_num_counters(vcpu);

> Why can't the value of vcpu->kvm->arch.nr_pmu_counters be trusted?

Similar to discussion on a previous patch, I need to move some
validation to the ioctl where it's set.

>> @@ -1360,7 +1360,7 @@ static int set_pmcr(struct kvm_vcpu *vcpu, const  
>> struct sys_reg_desc *r,
>>   	 */
>>   	if (!kvm_vm_has_ran_once(kvm) &&
>>   	    !vcpu_has_nv(vcpu)	      &&
>> -	    new_n <= kvm_arm_pmu_get_max_counters(kvm))
>> +	    new_n <= kvm_pmu_hpmn(vcpu))
>>   		kvm->arch.nr_pmu_counters = new_n;

> This is the legacy UAPI for setting the number of PMU counters by
> writing to PMCR_EL0.N.

> The 'partitioned' implementation should take a dependency on the
> SET_NR_COUNTERS attribute and reject attempts to change the value of
> PMCR_EL0.N. Just like nested.

Good! I hated making writes to PMCR_EL0.N work and only did it because
of selftests assuming it did.


> Thanks,
> Oliver