Re: [RFC PATCH 09/41] perf: core/x86: Forbid PMI handler when guest own PMU

[Sean Christopherson]

On Fri, Jan 26, 2024, Xiong Zhang wrote:
> +	/*
> +	 * When PMU is pass-through into guest, this handler should be forbidden from
> +	 * running, the reasons are:
> +	 * 1. After perf_guest_switch_to_kvm_pmi_vector() is called, and before cpu
> +	 *    enter into non-root mode, NMI could happen, but x86_pmu_handle_irq()
> +	 *    restore PMU to use NMI vector, which destroy KVM PMI vector setting.
> +	 * 2. When VM is running, host NMI other than PMI causes VM exit, KVM will
> +	 *    call host NMI handler (vmx_vcpu_enter_exit()) first before KVM save
> +	 *    guest PMU context (kvm_pmu_save_pmu_context()), as x86_pmu_handle_irq()
> +	 *    clear global_status MSR which has guest status now, then this destroy
> +	 *    guest PMU status.
> +	 * 3. After VM exit, but before KVM save guest PMU context, host NMI other
> +	 *    than PMI could happen, x86_pmu_handle_irq() clear global_status MSR
> +	 *    which has guest status now, then this destroy guest PMU status.
> +	 */
> +	if (perf_is_in_guest_passthrough())

Maybe a name more along the lines of:

	if (perf_is_guest_context_loaded())

because that makes it more obvious that the NMI _can't_ belong to the host PMU.

For that matter, I would also rename __perf_force_exclude_guest to
perf_guest_context_loaded (or "active" if that's better).  The boolean tracks
the state (guest vs. host context loaded/active), where as forcing perf events
to exclude_guest is an action based on that state.