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Subject: Re: [PATCH 4/5] perf arm-spe: Implement find_snapshot callback
To:     Leo Yan <leo.yan@linaro.org>
Cc:     linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        John Garry <john.garry@huawei.com>,
        Will Deacon <will@kernel.org>,
        Mathieu Poirier <mathieu.poirier@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Mike Leach <mike.leach@linaro.org>,
        linux-arm-kernel@lists.infradead.org, coresight@lists.linaro.org,
        James Clark <James.Clark@arm.com>
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 <20210923135016.GG400258@leoy-ThinkPad-X240s>
 <20210923144048.GB603008@leoy-ThinkPad-X240s>
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 <20211004122724.GC174271@leoy-ThinkPad-X240s>
 <6b092f13-832f-5d1d-a504-aea96c81bf17@arm.com>
 <20211006095124.GC14400@leoy-ThinkPad-X240s>
From:   German Gomez <german.gomez@arm.com>
Message-ID: <377b54ef-b9c0-9cfc-ef0c-0187d7c493cc@arm.com>
Date:   Mon, 11 Oct 2021 16:55:37 +0100
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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?

Many thanks,
German