linux-kernel - Re: [PATCH 10/17] KVM: arm64: Writethrough trapped PMEVTYPER register

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Message-ID: <gsntqzzz1d4o.fsf@coltonlewis-kvm.c.googlers.com>
Date: Wed, 04 Jun 2025 20:10:47 +0000
From: Colton Lewis <coltonlewis@...gle.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: kvm@...r.kernel.org, pbonzini@...hat.com, corbet@....net, 
	linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org, 
	maz@...nel.org, joey.gouly@....com, suzuki.poulose@....com, 
	yuzenghui@...wei.com, mark.rutland@....com, shuah@...nel.org, 
	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 10/17] KVM: arm64: Writethrough trapped PMEVTYPER register

Oliver Upton <oliver.upton@...ux.dev> writes:

> On Mon, Jun 02, 2025 at 07:26:55PM +0000, Colton Lewis wrote:
>> With FGT in place, the remaining trapped registers need to be written
>> through to the underlying physical registers as well as the virtual
>> ones. Failing to do this means delaying when guest writes take effect.

>> Signed-off-by: Colton Lewis <coltonlewis@...gle.com>
>> ---
>>   arch/arm64/kvm/sys_regs.c | 27 +++++++++++++++++++++++++--
>>   1 file changed, 25 insertions(+), 2 deletions(-)

>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index d368eeb4f88e..afd06400429a 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -18,6 +18,7 @@
>>   #include <linux/printk.h>
>>   #include <linux/uaccess.h>
>>   #include <linux/irqchip/arm-gic-v3.h>
>> +#include <linux/perf/arm_pmu.h>
>>   #include <linux/perf/arm_pmuv3.h>

>>   #include <asm/arm_pmuv3.h>
>> @@ -942,7 +943,11 @@ static bool pmu_counter_idx_valid(struct kvm_vcpu  
>> *vcpu, u64 idx)
>>   {
>>   	u64 pmcr, val;

>> -	pmcr = kvm_vcpu_read_pmcr(vcpu);
>> +	if (kvm_vcpu_pmu_is_partitioned(vcpu))
>> +		pmcr = read_pmcr();

> Reading PMCR_EL0 from EL2 is not going to have the desired effect.
> PMCR_EL0.N only returns HPMN when read from the guest.

Okay. I'll change that.

>> +	else
>> +		pmcr = kvm_vcpu_read_pmcr(vcpu);
>> +
>>   	val = FIELD_GET(ARMV8_PMU_PMCR_N, pmcr);
>>   	if (idx >= val && idx != ARMV8_PMU_CYCLE_IDX) {
>>   		kvm_inject_undefined(vcpu);
>> @@ -1037,6 +1042,22 @@ static bool access_pmu_evcntr(struct kvm_vcpu  
>> *vcpu,
>>   	return true;
>>   }

>> +static void writethrough_pmevtyper(struct kvm_vcpu *vcpu, struct  
>> sys_reg_params *p,
>> +				   u64 reg, u64 idx)
>> +{
>> +	u64 evmask = kvm_pmu_evtyper_mask(vcpu->kvm);
>> +	u64 val = p->regval & evmask;
>> +
>> +	__vcpu_sys_reg(vcpu, reg) = val;
>> +
>> +	if (idx == ARMV8_PMU_CYCLE_IDX)
>> +		write_pmccfiltr(val);
>> +	else if (idx == ARMV8_PMU_INSTR_IDX)
>> +		write_pmicfiltr(val);
>> +	else
>> +		write_pmevtypern(idx, val);
>> +}
>> +

> How are you preventing the VM from configuring an event counter to count
> at EL2?

I had thought that's what kvm_pmu_evtyper_mask() did since masking with
that is what kvm_pmu_set_counter_event_type() writes to the vCPU register.

> I see that you're setting MDCR_EL2.HPMD (which assumes FEAT_PMUv3p1) but
> due to an architecture bug there's no control to prohibit the cycle
> counter until FEAT_PMUv3p5 (MDCR_EL2.HCCD).

I'll fix that.

> Since you're already trapping PMCCFILTR you could potentially configure
> the hardware value in such a way that it filters EL2.

Sure.

>>   static bool access_pmu_evtyper(struct kvm_vcpu *vcpu, struct  
>> sys_reg_params *p,
>>   			       const struct sys_reg_desc *r)
>>   {
>> @@ -1063,7 +1084,9 @@ static bool access_pmu_evtyper(struct kvm_vcpu  
>> *vcpu, struct sys_reg_params *p,
>>   	if (!pmu_counter_idx_valid(vcpu, idx))
>>   		return false;

>> -	if (p->is_write) {
>> +	if (kvm_vcpu_pmu_is_partitioned(vcpu) && p->is_write) {
>> +		writethrough_pmevtyper(vcpu, p, reg, idx);

> What about the vPMU event filter?

I'll check that too.

>> +	} else if (p->is_write) {
>>   		kvm_pmu_set_counter_event_type(vcpu, p->regval, idx);
>>   		kvm_vcpu_pmu_restore_guest(vcpu);
>>   	} else {
>> --
>> 2.49.0.1204.g71687c7c1d-goog


> Thanks,
> Oliver