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

[Sean Christopherson]

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.