Re: [PATCH v4 29/38] KVM: x86/pmu: Switch host/guest PMU context at vm-exit/vm-entry

[Mi, Dapeng]

On 5/16/2025 9:26 PM, Sean Christopherson wrote:
> On Mon, Mar 24, 2025, Mingwei Zhang wrote:
>> +	/*
>> +	 * Clear hardware selector MSR content and its counter to avoid
>> +	 * leakage and also avoid this guest GP counter get accidentally
>> +	 * enabled during host running when host enable global ctrl.
>> +	 */
>> +	for (i = 0; i < pmu->nr_arch_gp_counters; i++) {
>> +		pmc = &pmu->gp_counters[i];
>> +		eventsel_msr = pmc_msr_addr(pmu, pmu->gp_eventsel_base, i);
>> +		counter_msr = pmc_msr_addr(pmu, pmu->gp_counter_base, i);
>> +
>> +		rdpmcl(i, pmc->counter);
>> +		rdmsrl(eventsel_msr, pmc->eventsel_hw);
> As pointed out by Dapeng offlist, this RDMSR is unnecessary since event selector
> MSRs are always intercepted.
>
>> +		if (pmc->counter)
>> +			wrmsrl(counter_msr, 0);
>> +		if (pmc->eventsel_hw)
>> +			wrmsrl(eventsel_msr, 0);
>> +	}
>> +
>> +	for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
>> +		pmc = &pmu->fixed_counters[i];
>> +		counter_msr = pmc_msr_addr(pmu, pmu->fixed_base, i);
>> +
>> +		rdpmcl(INTEL_PMC_FIXED_RDPMC_BASE | i, pmc->counter);
>> +		if (pmc->counter)
>> +			wrmsrl(counter_msr, 0);
>> +	}
>> +
>> +}
>> +static void intel_put_guest_context(struct kvm_vcpu *vcpu)
>> +{
>> +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>> +
>> +	/* Global ctrl register is already saved at VM-exit. */
>> +	rdmsrl(MSR_CORE_PERF_GLOBAL_STATUS, pmu->global_status);
>> +
>> +	/* Clear hardware MSR_CORE_PERF_GLOBAL_STATUS MSR, if non-zero. */
>> +	if (pmu->global_status)
>> +		wrmsrl(MSR_CORE_PERF_GLOBAL_OVF_CTRL, pmu->global_status);
>> +
>> +	rdmsrl(MSR_CORE_PERF_FIXED_CTR_CTRL, pmu->fixed_ctr_ctrl_hw);
> And same thing here.  Though I'm confused as to why KVM always intercept
> FIXED_CTR_CTRL.
>
> /me rummages around the SDM
>
> Ahh, because there are Any Thread bits in there.  That absolutely needs to be
> called out, probably in the interception logic in pmu_intel.c.  I'll add a comment.

Another reason is event filter. User may select to filter out some but not
all fixed counters, so PERF_FIXED_CTR_CTRL has to be intercepted.


>