Re: [PATCH v4 27/38] KVM: x86/pmu: Handle PMU MSRs interception and event filtering

[Sean Christopherson <seanjc@google.com>]

Again, be more precise.

On Mon, Mar 24, 2025, Mingwei Zhang wrote:
> From: Dapeng Mi <dapeng1.mi@linux.intel.com>
> 
> Mediated vPMU needs to intercept EVENTSELx and FIXED_CNTR_CTRL MSRs to
> filter out guest malicious perf events. Either writing these MSRs or
> updating event filters would call reprogram_counter() eventually. Thus
> check if the guest event should be filtered out in reprogram_counter().
> If so, clear corresponding EVENTSELx MSR or FIXED_CNTR_CTRL field to
> ensure the guest event won't be really enabled at vm-entry.
> 
> Besides, mediated vPMU intercepts the MSRs of these guest not owned
> counters and it just needs simply to read/write from/to pmc->counter.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> Co-developed-by: Mingwei Zhang <mizhang@google.com>
> Signed-off-by: Mingwei Zhang <mizhang@google.com>
> ---
>  arch/x86/kvm/pmu.c | 27 +++++++++++++++++++++++++++
>  arch/x86/kvm/pmu.h |  3 +++
>  2 files changed, 30 insertions(+)
> 
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 63143eeb5c44..e9100dc49fdc 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -305,6 +305,11 @@ static void pmc_update_sample_period(struct kvm_pmc *pmc)
>  
>  void pmc_write_counter(struct kvm_pmc *pmc, u64 val)
>  {
> +	if (kvm_mediated_pmu_enabled(pmc->vcpu)) {
> +		pmc->counter = val & pmc_bitmask(pmc);
> +		return;
> +	}
> +
>  	/*
>  	 * Drop any unconsumed accumulated counts, the WRMSR is a write, not a
>  	 * read-modify-write.  Adjust the counter value so that its value is
> @@ -455,6 +460,28 @@ static int reprogram_counter(struct kvm_pmc *pmc)
>  	bool emulate_overflow;
>  	u8 fixed_ctr_ctrl;
>  
> +	if (kvm_mediated_pmu_enabled(pmu_to_vcpu(pmu))) {
> +		bool allowed = check_pmu_event_filter(pmc);
> +
> +		if (pmc_is_gp(pmc)) {
> +			if (allowed)
> +				pmc->eventsel_hw |= pmc->eventsel &
> +						    ARCH_PERFMON_EVENTSEL_ENABLE;
> +			else
> +				pmc->eventsel_hw &= ~ARCH_PERFMON_EVENTSEL_ENABLE;
> +		} else {
> +			int idx = pmc->idx - KVM_FIXED_PMC_BASE_IDX;
> +
> +			if (allowed)
> +				pmu->fixed_ctr_ctrl_hw = pmu->fixed_ctr_ctrl;
> +			else
> +				pmu->fixed_ctr_ctrl_hw &=
> +					~intel_fixed_bits_by_idx(idx, 0xf);
> +		}
> +
> +		return 0;

I think it's worth adding a helper for this, as it makes things a bit more
self-documenting in terms of when KVM needs to "reprogram" mediated PMU PMCs.