Re: [PATCH 7/9] perf/x86/intel: Disable sampling read slots and topdown

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 21, 2019 at 02:40:53PM -0700, kan.liang@linux.intel.com wrote:
> From: Kan Liang <kan.liang@linux.intel.com>
> 
> To get correct PERF_METRICS value, the fixed counter 3 must start from
> 0. It would bring problems when sampling read slots and topdown events.
> For example,
>         perf record -e '{slots, topdown-retiring}:S'
> The slots would not overflow if it starts from 0.
> 
> Add specific validate_group() support to reject the case and error out
> for Icelake.
> 
> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
> ---
>  arch/x86/events/core.c       |  2 ++
>  arch/x86/events/intel/core.c | 20 ++++++++++++++++++++
>  arch/x86/events/perf_event.h |  2 ++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 07ecfe75f0e6..a7eb842f8651 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -2065,6 +2065,8 @@ static int validate_group(struct perf_event *event)
>  	fake_cpuc->n_events = 0;
>  	ret = x86_pmu.schedule_events(fake_cpuc, n, NULL);
>  
> +	if (x86_pmu.validate_group)
> +		ret = x86_pmu.validate_group(fake_cpuc, n);
>  out:
>  	free_fake_cpuc(fake_cpuc);
>  	return ret;
> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> index 79e9d05e047d..2bb90d652a35 100644
> --- a/arch/x86/events/intel/core.c
> +++ b/arch/x86/events/intel/core.c
> @@ -4410,6 +4410,25 @@ static int icl_set_period(struct perf_event *event)
>  	return 1;
>  }
>  
> +static int icl_validate_group(struct cpu_hw_events *cpuc, int n)
> +{
> +	bool has_sampling_slots = false, has_metrics = false;
> +	struct perf_event *e;
> +	int i;
> +
> +	for (i = 0; i < n; i++) {
> +		e = cpuc->event_list[i];
> +		if (is_slots_event(e) && is_sampling_event(e))
> +			has_sampling_slots = true;
> +
> +		if (is_perf_metrics_event(e))
> +			has_metrics = true;
> +	}
> +	if (unlikely(has_sampling_slots && has_metrics))
> +		return -EINVAL;
> +	return 0;
> +}

Why this special hack, why not disallow sampling on SLOTS on creation?