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

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.