Re: [RESEND PATCH V5 1/2] perf/core: Add new branch sample type for HW index of raw branch records

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 16, 2020 at 07:57:56AM -0800, kan.liang@linux.intel.com wrote:

>  struct perf_branch_stack {
>  	__u64				nr;
> +	__u64				hw_idx;
>  	struct perf_branch_entry	entries[0];
>  };

The above and below order doesn't match.

> @@ -849,7 +853,11 @@ enum perf_event_type {
>  	 *	  char                  data[size];}&& PERF_SAMPLE_RAW
>  	 *
>  	 *	{ u64                   nr;
> -	 *        { u64 from, to, flags } lbr[nr];} && PERF_SAMPLE_BRANCH_STACK
> +	 *        { u64 from, to, flags } lbr[nr];
> +	 *
> +	 *        # only available if PERF_SAMPLE_BRANCH_HW_INDEX is set
> +	 *        u64			hw_idx;
> +	 *      } && PERF_SAMPLE_BRANCH_STACK

That wants to be written as:

		{ u64			nr;
		  { u64 from, to, flags; } entries[nr];
		  { u64	hw_idx; } && PERF_SAMPLE_BRANCH_HW_INDEX
		} && PERF_SAMPLE_BRANCH_STACK

But the big question is; why isn't it:

		{ u64			nr;
		  { u64	hw_idx; } && PERF_SAMPLE_BRANCH_HW_INDEX
		  { u64 from, to, flags; } entries[nr];
		} && PERF_SAMPLE_BRANCH_STACK

to match the struct perf_branch_stack order. Having that variable sized
entry in the middle just seems weird.

>  	 *
>  	 * 	{ u64			abi; # enum perf_sample_regs_abi
>  	 * 	  u64			regs[weight(mask)]; } && PERF_SAMPLE_REGS_USER