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Subject: Re: [PATCH v1 2/3] perf session: Add swap operation for event
 TIME_CONV
To:     Leo Yan <leo.yan@linaro.org>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Steve MacLean <Steve.MacLean@Microsoft.com>,
        Yonatan Goldschmidt <yonatan.goldschmidt@granulate.io>,
        Kan Liang <kan.liang@linux.intel.com>,
        linux-kernel@vger.kernel.org
References: <20210412083459.462817-1-leo.yan@linaro.org>
 <20210412083459.462817-3-leo.yan@linaro.org>
 <1b4fa7c3-79cd-6f73-873e-4792b8a1b732@intel.com>
 <20210426064421.GA327854@leoy-ThinkPad-X240s>
From:   Adrian Hunter <adrian.hunter@intel.com>
Organization: Intel Finland Oy, Registered Address: PL 281, 00181 Helsinki,
 Business Identity Code: 0357606 - 4, Domiciled in Helsinki
Message-ID: <7f21edac-1bd9-ab98-aeca-2109e5edf5c3@intel.com>
Date:   Mon, 26 Apr 2021 10:26:12 +0300
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On 26/04/21 9:44 am, Leo Yan wrote:
> Hi Adrian,
> 
> On Mon, Apr 26, 2021 at 08:40:00AM +0300, Adrian Hunter wrote:
> 
> [...]
> 
>>> +static void perf_event__time_conv_swap(union perf_event *event,
>>> +				       bool sample_id_all __maybe_unused)
>>> +{
>>> +	size_t time_zero_size;
>>> +
>>> +	event->time_conv.time_shift = bswap_64(event->time_conv.time_shift);
>>> +	event->time_conv.time_mult  = bswap_64(event->time_conv.time_mult);
>>> +	event->time_conv.time_zero  = bswap_64(event->time_conv.time_zero);
>>> +
>>> +	time_zero_size = (void *)&event->time_conv.time_cycles - (void *)event;
>>> +	if (event->header.size > time_zero_size) {
>>
>> I wonder if we could have a helper for this e.g. (untested)
>>
>> #define event_contains(obj, mem) (obj.header.size > offsetof(typeof(obj), mem))
>>
>> 	if (event_contains(event->time_conv, time_cycles)) {
> 
> Yeah, this is a better implementation.  Will refine patch for this.
> 
>>> +		event->time_conv.time_cycles = bswap_64(event->time_conv.time_cycles);
>>> +		event->time_conv.time_mask = bswap_64(event->time_conv.time_mask);
>>> +		event->time_conv.cap_user_time_zero =
>>> +			bswap_32(event->time_conv.cap_user_time_zero);
>>> +		event->time_conv.cap_user_time_short =
>>> +			bswap_32(event->time_conv.cap_user_time_short);
>>
>> 'struct perf_record_time_conv' contains bool, the sizeof which, AFAIK, is not defined.
>> Is it really 4 bytes on your implementation?  It is only 1 byte with gcc on x86.
> 
> On Arm64, bool is also 1 byte with GCC.
> 
> When working on this patch, I checked the size for structure
> perf_record_time_conv, it gave out the structure size is 56; I wrongly
> thought the bool size is 4 bytes and there have no padding.  In fact,
> bool size is 1 byte and GCC pads 6 bytes at the end of structure.
> 
>> Either way, you should change 'struct perf_record_time_conv' so it uses a type of known size.
>> Since you are the only one using it, it should match your implementation.
> 
> Will change to "unsigned int" value.

If it is 1 byte, I'd have thought __u8 i.e.

struct perf_record_time_conv {
	struct perf_event_header header;
	__u64			 time_shift;
	__u64			 time_mult;
	__u64			 time_zero;
	__u64			 time_cycles;
	__u64			 time_mask;
	__u8			 cap_user_time_zero;
	__u8			 cap_user_time_short;
	__u8			 unused[6];
};