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

[Jiri Olsa <jolsa@redhat.com>]

On Thu, Sep 30, 2021 at 12:34:09PM +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__reverse64_branch_stack_flags()
> is defined and introduced in evsel__parse_sample.
> 
> Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
> ---
>  tools/perf/util/evsel.c | 64 +++++++++++++++++++++++++++++++++++++++--
>  tools/perf/util/evsel.h |  5 ++++
>  2 files changed, 67 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index dbfeceb2546c..007be66b69a2 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -2221,6 +2221,46 @@ void __weak arch_perf_parse_sample_weight(struct perf_sample *data,
>  	data->weight = *array;
>  }
>  
> +u64 evsel__reverse64_branch_stack_flags(u64 value)
> +{
> +	u64 new_val = 0;
> +
> +	/*
> +	 * branch_stack flag (u64)
> +	 * 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
> +	 * 	}
> +	 * }
> +	 */

please describe in comment how the bitfield is swapped


> +	if (bigendian()) {
> +		new_val = reverse_64(value, 0, 1);
> +		new_val |= reverse_64(value, 1, 1);
> +		new_val |= reverse_64(value, 2, 1);
> +		new_val |= reverse_64(value, 3, 1);
> +		new_val |= reverse_64(value, 4, 16);
> +		new_val |= reverse_64(value, 20, 4);
> +		new_val |= reverse_64(value, 24, 40);
> +	} else {
> +		new_val = reverse_64(value, 63, 1);
> +		new_val |= reverse_64(value, 62, 1);
> +		new_val |= reverse_64(value, 61, 1);
> +		new_val |= reverse_64(value, 60, 1);
> +		new_val |= reverse_64(value, 44, 16);
> +		new_val |= reverse_64(value, 40, 4);
> +		new_val |= reverse_64(value, 0, 40);
> +	}
> +
> +	return new_val;
> +}
> +
>  int evsel__parse_sample(struct evsel *evsel, union perf_event *event,
>  			struct perf_sample *data)
>  {
> @@ -2408,6 +2448,8 @@ 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 +2458,28 @@ 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;
> +			e = (struct branch_entry *)&data->branch_stack->hw_idx;

hum, why do we convert hw_idx? it's the same struct as entries?
please explain this in comment as well

> +		}
> +
> +		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__reverse64_branch_stack_flags() uses a reverse64
> +			 * macro to reverse the bit position based on the host
> +			 * endians.
> +			 */
> +			for (i = 0; i < data->branch_stack->nr; i++, e++)
> +				e->flags.value = evsel__reverse64_branch_stack_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..1127c23710cf 100644
> --- a/tools/perf/util/evsel.h
> +++ b/tools/perf/util/evsel.h
> @@ -482,4 +482,9 @@ 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);
> +
> +#define reverse_64(src, pos, size)	\
> +	((((src) >> (pos)) & ((1ull << (size)) - 1)) << (63 - ((pos) + (size) - 1)))

hum, is this reversing anything?
could you please add comment describing what this is doing?

thanks,
jirka

> +
> +u64 evsel__reverse64_branch_stack_flags(u64 value);
>  #endif /* __PERF_EVSEL_H */
> -- 
> 2.31.1
>