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 Mon, Jan 20, 2020 at 11:50:59AM -0500, Liang, Kan wrote:
> 
> 
> On 1/20/2020 4:23 AM, Peter Zijlstra wrote:
> > 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.
> 
> 
> Usually, new data should be output to the end of a sample.

Because.... you want old tools to read new output?

> However, the entries[0] is sized entry, so I have to put the hw_idx before

entries[0] is only in the C thing, and in C you indeed have to put
hw_idx before.

> entry. It makes the inconsistency. Sorry for the confusion caused.

n/p it's clear now I think.