Messages in this thread |  | | Subject | Re: [PATCH 4/5] perf arm-spe: Implement find_snapshot callback | From | German Gomez <> | Date | Thu, 30 Sep 2021 13:26:15 +0100 |
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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
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