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Subject: Re: [PATCH v2 1/2] tools/perf: Add bitfield_swap to handle
 branch_stack endian issue
To:     Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>
Cc:     jolsa@redhat.com, michael@ellerman.id.au, eranian@google.com,
        mark.rutland@arm.com, namhyung@kernel.org, kjain@linux.ibm.com,
        atrajeev@linux.vnet.ibm.com, linux-perf-users@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20211016125059.691856-1-maddy@linux.ibm.com>
 <YXnr8xpltsVZPATC@kernel.org>
 <FD195114-27C2-4F8F-B05E-2E9D5595C4EF@gmail.com>
From:   Madhavan Srinivasan <maddy@linux.ibm.com>
Message-ID: <31b42a66-3e56-ab72-b2cb-798955c367e0@linux.ibm.com>
Date:   Thu, 28 Oct 2021 12:49:45 +0530
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On 10/28/21 8:07 AM, Arnaldo Carvalho de Melo wrote:
>
> On October 27, 2021 9:16:51 PM GMT-03:00, Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
>> Em Sat, Oct 16, 2021 at 06:20:58PM +0530, Madhavan Srinivasan escreveu:
>>> 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>
>>> ---
>>> Changelog v1:
>>> - Renamed function and macro
>>> - Added comments in code
> Please, run 'perf test' before submitting patches, it's not that difficult, please do so.
>
> - Arnaldo
>
>> [acme@quaco perf]$ perf test python
>> 19: 'import perf' in python                                         : FAILED!
>> [acme@quaco perf]$ perf test -v python
>> Couldn't bump rlimit(MEMLOCK), failures may take place when creating BPF maps, etc
>> 19: 'import perf' in python                                         :
>> --- start ---
>> test child forked, pid 991284
>> python usage test: "echo "import sys ; sys.path.append('/tmp/build/perf/python'); import perf" | '/usr/bin/python3' "
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>> ImportError: /tmp/build/perf/python/perf.cpython-39-x86_64-linux-gnu.so: undefined symbol: bigendian
>> test child finished with -1

using tep_is_bigendian() instead of bigendian() fixes it.
Will send a V3 with that changes.

Maddy

>> ---- end ---- 'import perf' in python: FAILED! [acme@quaco perf]$
>>>   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