Re: [PATCH v1 2/3] perf session: Add swap operation for event TIME_CONV

[Adrian Hunter <adrian.hunter@intel.com>]

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];
};