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Date:   Tue, 12 Oct 2021 09:19:48 +0100
From:   Will Deacon <will@kernel.org>
To:     German Gomez <german.gomez@arm.com>
Cc:     Leo Yan <leo.yan@linaro.org>, linux-kernel@vger.kernel.org,
        linux-perf-users@vger.kernel.org,
        John Garry <john.garry@huawei.com>,
        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>
Subject: Re: [PATCH 4/5] perf arm-spe: Implement find_snapshot callback
Message-ID: <20211012081948.GA5156@willie-the-truck>
References: <20210916154635.1525-1-german.gomez@arm.com>
 <20210916154635.1525-4-german.gomez@arm.com>
 <20210923135016.GG400258@leoy-ThinkPad-X240s>
 <20210923144048.GB603008@leoy-ThinkPad-X240s>
 <1c6a3a73-27dc-6673-7fe7-34bc7fcb0a68@arm.com>
 <20211004122724.GC174271@leoy-ThinkPad-X240s>
 <6b092f13-832f-5d1d-a504-aea96c81bf17@arm.com>
 <20211006095124.GC14400@leoy-ThinkPad-X240s>
 <377b54ef-b9c0-9cfc-ef0c-0187d7c493cc@arm.com>
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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?

Will