Re: [PATCH v2 1/2] tools/perf: Add bitfield_swap to handle branch_stack endian issue

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Mon, Oct 25, 2021 at 02:07:23PM +0200, Jiri Olsa escreveu:
> On Sat, Oct 16, 2021 at 06:20:58PM +0530, Madhavan Srinivasan wrote:
> > branch_stack struct has bit field definition which
> > produces different bit ordering for big/little endian.
> > Because of this, when branch_stack sample is collected
> > in a BE system and viewed/reported in a LE system, bit
> > fields of the branch stack are not presented properly.
> > To address this issue, a evsel__bitfield_swap_branch_stack()
> > is defined and introduced in evsel__parse_sample.
> > 
> > Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
> 
> for both patches
> 
> Acked-by: Jiri Olsa <jolsa@redhat.com>

Thanks, applied.

- Arnaldo

 
> thanks,
> jirka
> 
> > ---
> > Changelog v1:
> > - Renamed function and macro
> > - Added comments in code
> > 
> >  tools/perf/util/evsel.c | 74 +++++++++++++++++++++++++++++++++++++++--
> >  tools/perf/util/evsel.h | 13 ++++++++
> >  2 files changed, 85 insertions(+), 2 deletions(-)
> > 
> > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > index dbfeceb2546c..746e642d4d32 100644
> > --- a/tools/perf/util/evsel.c
> > +++ b/tools/perf/util/evsel.c
> > @@ -2221,6 +2221,51 @@ void __weak arch_perf_parse_sample_weight(struct perf_sample *data,
> >  	data->weight = *array;
> >  }
> >  
> > +u64 evsel__bitfield_swap_branch_flags(u64 value)
> > +{
> > +	u64 new_val = 0;
> > +
> > +	/*
> > +	 * branch_flags
> > +	 * union {
> > +	 * 	u64 values;
> > +	 * 	struct {
> > +	 * 		mispred:1	//target mispredicted
> > +	 * 		predicted:1	//target predicted
> > +	 * 		in_tx:1		//in transaction
> > +	 * 		abort:1		//transaction abort
> > +	 * 		cycles:16	//cycle count to last branch
> > +	 * 		type:4		//branch type
> > +	 * 		reserved:40
> > +	 * 	}
> > +	 * }
> > +	 *
> > +	 * Avoid bswap64() the entire branch_flag.value,
> > +	 * as it has variable bit-field sizes. Instead the
> > +	 * macro takes the bit-field position/size,
> > +	 * swaps it based on the host endianness.
> > +	 */
> > +	if (bigendian()) {
> > +		new_val = bitfield_swap(value, 0, 1);
> > +		new_val |= bitfield_swap(value, 1, 1);
> > +		new_val |= bitfield_swap(value, 2, 1);
> > +		new_val |= bitfield_swap(value, 3, 1);
> > +		new_val |= bitfield_swap(value, 4, 16);
> > +		new_val |= bitfield_swap(value, 20, 4);
> > +		new_val |= bitfield_swap(value, 24, 40);
> > +	} else {
> > +		new_val = bitfield_swap(value, 63, 1);
> > +		new_val |= bitfield_swap(value, 62, 1);
> > +		new_val |= bitfield_swap(value, 61, 1);
> > +		new_val |= bitfield_swap(value, 60, 1);
> > +		new_val |= bitfield_swap(value, 44, 16);
> > +		new_val |= bitfield_swap(value, 40, 4);
> > +		new_val |= bitfield_swap(value, 0, 40);
> > +	}
> > +
> > +	return new_val;
> > +}
> > +
> >  int evsel__parse_sample(struct evsel *evsel, union perf_event *event,
> >  			struct perf_sample *data)
> >  {
> > @@ -2408,6 +2453,8 @@ int evsel__parse_sample(struct evsel *evsel, union perf_event *event,
> >  	if (type & PERF_SAMPLE_BRANCH_STACK) {
> >  		const u64 max_branch_nr = UINT64_MAX /
> >  					  sizeof(struct branch_entry);
> > +		struct branch_entry *e;
> > +		unsigned int i;
> >  
> >  		OVERFLOW_CHECK_u64(array);
> >  		data->branch_stack = (struct branch_stack *)array++;
> > @@ -2416,10 +2463,33 @@ int evsel__parse_sample(struct evsel *evsel, union perf_event *event,
> >  			return -EFAULT;
> >  
> >  		sz = data->branch_stack->nr * sizeof(struct branch_entry);
> > -		if (evsel__has_branch_hw_idx(evsel))
> > +		if (evsel__has_branch_hw_idx(evsel)) {
> >  			sz += sizeof(u64);
> > -		else
> > +			e = &data->branch_stack->entries[0];
> > +		} else {
> >  			data->no_hw_idx = true;
> > +			/*
> > +			 * if the PERF_SAMPLE_BRANCH_HW_INDEX is not applied,
> > +			 * only nr and entries[] will be output by kernel.
> > +			 */
> > +			e = (struct branch_entry *)&data->branch_stack->hw_idx;
> > +		}
> > +
> > +		if (swapped) {
> > +			/*
> > +			 * struct branch_flag does not have endian
> > +			 * specific bit field definition. And bswap
> > +			 * will not resolve the issue, since these
> > +			 * are bit fields.
> > +			 *
> > +			 * evsel__bitfield_swap_branch_flags() uses a
> > +			 * bitfield_swap macro to swap the bit position
> > +			 * based on the host endians.
> > +			 */
> > +			for (i = 0; i < data->branch_stack->nr; i++, e++)
> > +				e->flags.value = evsel__bitfield_swap_branch_flags(e->flags.value);
> > +		}
> > +
> >  		OVERFLOW_CHECK(array, sz, max_size);
> >  		array = (void *)array + sz;
> >  	}
> > diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
> > index 1f7edfa8568a..2e82cdbe2c08 100644
> > --- a/tools/perf/util/evsel.h
> > +++ b/tools/perf/util/evsel.h
> > @@ -482,4 +482,17 @@ struct evsel *evsel__leader(struct evsel *evsel);
> >  bool evsel__has_leader(struct evsel *evsel, struct evsel *leader);
> >  bool evsel__is_leader(struct evsel *evsel);
> >  void evsel__set_leader(struct evsel *evsel, struct evsel *leader);
> > +
> > +/*
> > + * Macro to swap the bit-field postition and size.
> > + * Used when,
> > + * - dont need to swap the entire u64 &&
> > + * - when u64 has variable bit-field sizes &&
> > + * - when presented in a host endian which is different
> > + *   than the source endian of the perf.data file
> > + */
> > +#define bitfield_swap(src, pos, size)	\
> > +	((((src) >> (pos)) & ((1ull << (size)) - 1)) << (63 - ((pos) + (size) - 1)))
> > +
> > +u64 evsel__bitfield_swap_branch_flags(u64 value);
> >  #endif /* __PERF_EVSEL_H */
> > -- 
> > 2.31.1
> > 

-- 

- Arnaldo