Re: [PATCH 4/5] perf arm-spe: Implement find_snapshot callback

[German Gomez <german.gomez@arm.com>]

Hi Leo,

On 23/09/2021 15:40, Leo Yan wrote:
> On Thu, Sep 23, 2021 at 09:50:16PM +0800, Leo Yan wrote:
>
> [...]
>
>>> @@ -336,6 +480,7 @@ struct auxtrace_record *arm_spe_recording_init(int *err,
>>>   	sper->itr.pmu = arm_spe_pmu;
>>>   	sper->itr.snapshot_start = arm_spe_snapshot_start;
>>>   	sper->itr.snapshot_finish = arm_spe_snapshot_finish;
>>> +	sper->itr.find_snapshot = arm_spe_find_snapshot;
>> If I understand correctly, this patch copies the code from cs-etm for
>> snapshot handling.  About 2 months ago, we removed the Arm cs-etm's
>> specific snapshot callback function and directly use perf's function
>> __auxtrace_mmap__read() to handle 'head' and 'tail' pointers.  Please
>> see the commit for details:
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2f01c200d4405c4562e45e8bb4de44a5ce37b217
>>
>> Before I review more details for snapshot enabling in patches 03 and
>> 04, could you confirm if Arm SPE can use the same way with cs-etm for
>> snapshot handling?  From my understanding, this is a better way to
>> handle AUX buffer's 'head' and 'tail'.
> In other words, if we can only apply patch 03 and can pass the testing
> in patch 05, then it would be a very neat implementation.
>
> I will try to verify these patches and will get back result.
>
> Thanks,
> Leo
The patch is indeed based on that commit. The reason behind it is that the
values for *head are being wrapped in the driver side (see the macro
PERF_IDX2OFF which is used at various points in 
/drivers/perf/arm_spe_pmu.c).

If this callback is not to be added, I believe the driver needs to be 
updated
first so that the head pointer monotonically increases like in cs-etm. 
Do you
think this makes sense for SPE?

(note that the patch will skip the wrap-around detection if this is the 
case,
in order to handle both cases in the userspace perf tool).

Thanks,
German