Re: [RFC PATCH 07/17] perf: Add pmu_info to user page

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 05, 2017 at 04:30:16PM +0300, Alexander Shishkin wrote:
> Allow PMUs to supply additional static information that may be required
> by their decoders. Most of what Intel PT driver exports as capability
> attributes (timing packet freqencies, frequency ratios etc), its decoder
> needs to be able to correctly decode its binary stream. However, when
> decoding Intel PT stream from a core dump, we can't rely on the sysfs
> attributes, so we need to pack this information into the perf buffer,
> so that the resulting core dump is self-contained.
> 
> In order to do this, we append a PMU-specific structure to the user
> page. Such structures will include size, for versioning.
> 

> @@ -508,6 +513,18 @@ struct perf_addr_filters_head {
>  	unsigned int		nr_file_filters;
>  };
>  
> +struct pmu_info {
> +	/*
> +	 * Size of this structure, for versioning.
> +	 */
> +	u32	note_size;
> +
> +	/*
> +	 * Size of the container structure, not including this one
> +	 */
> +	u32	pmu_descsz;
> +};
> +
>  /**
>   * enum perf_event_active_state - the states of a event
>   */
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 3d64d9ea80..4cdd4fab9d 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -599,6 +599,16 @@ struct perf_event_mmap_page {
>  	__u64	aux_tail;
>  	__u64	aux_offset;
>  	__u64	aux_size;
> +
> +	/*
> +	 * PMU data: static info that (AUX) decoder wants to know in order to
> +	 * decode correctly:
> +	 *
> +	 *   pmu_offset >= sizeof(struct perf_event_mmap_page)
> +	 *   pmu_offset + pmu_size <= PAGE_SIZE
> +	 */
> +	__u64	pmu_offset;
> +	__u64	pmu_size;
>  };

Why like this? Why not dump the data as part of
PERF_RECORD_ITRACE_START/PERF_RECORD_AUX ?