Messages in this thread | | | Subject | Re: [PATCH 4/5] perf arm-spe: Implement find_snapshot callback | From | James Clark <> | Date | Tue, 12 Oct 2021 09:47:27 +0100 |
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On 12/10/2021 09:19, Will Deacon wrote: > On Mon, Oct 11, 2021 at 04:55:37PM +0100, German Gomez wrote: >> Hi Leo, >> >> On 06/10/2021 10:51, Leo Yan wrote: >>> On Wed, Oct 06, 2021 at 10:35:20AM +0100, German Gomez wrote: >>> >>> [...] >>> >>>>> So simply say, I think the head pointer monotonically increasing is >>>>> the right thing to do in Arm SPE driver. >>>> I will talk to James about how we can proceed on this. >>> Thanks! >> >> I took this offline with James and, though it looks possible to patch >> the SPE driver to have a monotonically increasing head pointer in order >> to simplify the handling in the perf tool, it could be a breaking change >> for users of the perf_event_open syscall that currently rely on the way >> it works now. >> >> An alternative way we considered to simplify the patch is to change the >> logic inside the find_snapshot callback so that it records the entire >> contents of the aux buffer every time. >> >> What do you think? > > What does intel-pt do?
Intel-pt has a wrapped head, which is why it has the intel_pt_find_snapshot() function in perf to try to not save any zeros from the buffer that haven't been written yet. (With a wrapped head pointer it's impossible to tell).
Coresight has a monotonically increasing head pointer so it is possible to tell. Recently, Leo removed the Coresight version of find_snapshot() for this reason.
It would be nice to do the same for SPE because that function has a heuristic and is also slow, but I imagine that not returning wrapped head pointers could break anything that expects them.
James
> > Will >
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