linux-kernel - Re: [PATCH 4/5] perf arm-spe: Implement find

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Date:   Tue, 12 Oct 2021 09:47:27 +0100
From:   James Clark <james.clark@....com>
To:     Will Deacon <will@...nel.org>, German Gomez <german.gomez@....com>
Cc:     Leo Yan <leo.yan@...aro.org>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org,
        John Garry <john.garry@...wei.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Mike Leach <mike.leach@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, coresight@...ts.linaro.org
Subject: Re: [PATCH 4/5] perf arm-spe: Implement find_snapshot callback



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
>