Messages in this thread | | | Date | Thu, 7 Oct 2021 19:42:07 +0200 | From | Jiri Olsa <> | Subject | Re: [PATCH 1/2] tools/perf: Add reverse_fn to handle branch_stack endian issue |
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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 >
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