Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI and inject it to guest

[Peter Zijlstra]

On Mon, Jan 04, 2021 at 09:15:29PM +0800, Like Xu wrote:

> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> index b47cc4226934..c499bdb58373 100644
> --- a/arch/x86/events/intel/ds.c
> +++ b/arch/x86/events/intel/ds.c
> @@ -1721,6 +1721,65 @@ intel_pmu_save_and_restart_reload(struct perf_event *event, int count)
>  	return 0;
>  }
>  
> +/*
> + * We may be running with guest PEBS events created by KVM, and the
> + * PEBS records are logged into the guest's DS and invisible to host.
> + *
> + * In the case of guest PEBS overflow, we only trigger a fake event
> + * to emulate the PEBS overflow PMI for guest PBES counters in KVM.
> + * The guest will then vm-entry and check the guest DS area to read
> + * the guest PEBS records.
> + *
> + * The guest PEBS overflow PMI may be dropped when both the guest and
> + * the host use PEBS. Therefore, KVM will not enable guest PEBS once
> + * the host PEBS is enabled since it may bring a confused unknown NMI.
> + *
> + * The contents and other behavior of the guest event do not matter.
> + */
> +static int intel_pmu_handle_guest_pebs(struct cpu_hw_events *cpuc,
> +				       struct pt_regs *iregs,
> +				       struct debug_store *ds)
> +{
> +	struct perf_sample_data data;
> +	struct perf_event *event = NULL;
> +	u64 guest_pebs_idxs = cpuc->pebs_enabled & ~cpuc->intel_ctrl_host_mask;
> +	int bit;
> +
> +	/*
> +	 * Ideally, we should check guest DS to understand if it's
> +	 * a guest PEBS overflow PMI from guest PEBS counters.
> +	 * However, it brings high overhead to retrieve guest DS in host.
> +	 * So we check host DS instead for performance.

Again; for the virt illiterate people here (me); why is it expensive to
check guest DS?

Why do we need to? Can't we simply always forward the PMI if the guest
has bits set in MSR_IA32_PEBS_ENABLE ? Surely we can access the guest
MSRs at a reasonable rate..

Sure, it'll send too many PMIs, but is that really a problem?

> +	 *
> +	 * If PEBS interrupt threshold on host is not exceeded in a NMI, there
> +	 * must be a PEBS overflow PMI generated from the guest PEBS counters.
> +	 * There is no ambiguity since the reported event in the PMI is guest
> +	 * only. It gets handled correctly on a case by case base for each event.
> +	 *
> +	 * Note: KVM disables the co-existence of guest PEBS and host PEBS.

Where; I need a code reference here.

> +	 */
> +	if (!guest_pebs_idxs || !in_nmi() ||

All the other code uses !iregs instead of !in_nmi(), also your
indentation is broken.

> +		ds->pebs_index >= ds->pebs_interrupt_threshold)
> +		return 0;
> +
> +	for_each_set_bit(bit, (unsigned long *)&guest_pebs_idxs,
> +			INTEL_PMC_IDX_FIXED + x86_pmu.num_counters_fixed) {
> +
> +		event = cpuc->events[bit];
> +		if (!event->attr.precise_ip)
> +			continue;
> +
> +		perf_sample_data_init(&data, 0, event->hw.last_period);
> +		if (perf_event_overflow(event, &data, iregs))
> +			x86_pmu_stop(event, 0);
> +
> +		/* Inject one fake event is enough. */
> +		return 1;
> +	}
> +
> +	return 0;
> +}
> +
>  static __always_inline void
>  __intel_pmu_pebs_event(struct perf_event *event,
>  		       struct pt_regs *iregs,
> @@ -1965,6 +2024,9 @@ static void intel_pmu_drain_pebs_icl(struct pt_regs *iregs, struct perf_sample_d
>  	if (!x86_pmu.pebs_active)
>  		return;
>  
> +	if (intel_pmu_handle_guest_pebs(cpuc, iregs, ds))
> +		return;
> +
>  	base = (struct pebs_basic *)(unsigned long)ds->pebs_buffer_base;
>  	top = (struct pebs_basic *)(unsigned long)ds->pebs_index;
>  
> -- 
> 2.29.2
>