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Date: Mon, 25 Oct 2021 17:02:47 +0530
From: Madhavan Srinivasan <maddy@...ux.ibm.com>
To: Jiri Olsa <jolsa@...hat.com>
Cc: acme@...nel.org, michael@...erman.id.au, eranian@...gle.com,
mark.rutland@....com, namhyung@...nel.org, kjain@...ux.ibm.com,
atrajeev@...ux.vnet.ibm.com, linux-perf-users@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] tools/perf: Add bitfield_swap to handle
branch_stack endian issue
On 10/21/21 6:55 PM, Jiri Olsa wrote:
> 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@...ux.ibm.com>
>> ---
>> 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) {
> thanks for all the comments, make it easy to review
>
>> + /*
>> + * 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);
> one last thing that confuses me.. the sample is already swapped at this
> point, right? with perf_event__all64_swap function
>
> should you swap it back first and then do the proper bitfield swap?
> like we do in PERF_SAMPLE_RAW to get proper u32 values
No, in case of PERF_SAMPLE_RAW type, first word contains the size.
Let me explain with an example (hope will not confuse further :) ),
say we have branch_flag.value as 0x4000000000000000 generated in a
BE system, which has predict set, now if we undo the swap done by
perf_event__all64_swap, value comes out as 0x40 (in a LE system) and
if this value fed to bitswap macro, we end up 0x0000000040000000,
which is wrong. Instead, by not swapping back, we feed the value
in the source-endian to the new macro.
Maddy
> thanks,
> jirka
>
>> + }
>> +
>> 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
>>
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