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From: Will Deacon <will@kernel.org>
To: Raphael Gault <raphael.gault@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	peterz@infradead.org, catalin.marinas@arm.com,
	will.deacon@arm.com,
	Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	linux-kernel@vger.kernel.org, mingo@redhat.com,
	mathieu.desnoyers@efficios.com,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 3/7] perf: arm64: Use rseq to test userspace access to pmu counters
Date: Tue, 25 Jun 2019 14:41:18 +0100
Message-ID: <20190625134117.r3gysn7mvzzdrytj@willie-the-truck> (raw)
In-Reply-To: <ff5897eb-ae6c-482f-148b-947a661fde4f@arm.com>

Hi Raphael,

On Tue, Jun 25, 2019 at 02:29:56PM +0100, Raphael Gault wrote:
> Now that we have a better idea what enabling this feature for heterogeneous
> systems would look like (both with or without using rseqs), it might be
> worth discussing if this is in fact a desirable thing in term of
> performance-complexity trade off.

Agreed; it's one of those things that I think is *definitely* worth
prototyping, but it's not obviously something we should commit to at this
stage.

> Indeed, while not as scary as first thought, maybe the rseq method would
> still dissuade users from using this feature. It is also worth noting that
> if we only enable this feature on homogeneous system, the `mrs`
> hook/emulation would not be necessary.
> If because of the complexity of the setup we need to consider whether we
> want to upstream this and have to maintain it afterward.

Given that we don't currently support userspace access to the counters at
all, I think upstreaming support only for homogeneous systems makes sense
initially as long as (a) we fail gracefully on heterogeneous systems and (b)
we don't close the door to using the rseq-based mechanism in future if we
choose to (i.e. it would be nice if this was an extension to the ABI rather
than a break). That also gives us a chance to assess the wider adoption of
rseq before throwing our weight behind it.

Sound reasonable?

Will

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Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-11 12:53 [PATCH 0/7] arm64: Enable access to pmu registers by user-space Raphael Gault
2019-06-11 12:53 ` [PATCH 1/7] perf: arm64: Compile tests unconditionally Raphael Gault
2019-06-11 14:09   ` Mark Rutland
2019-06-11 14:23     ` Arnaldo Carvalho de Melo
2019-06-11 17:01       ` Mark Rutland
2019-06-11 12:53 ` [PATCH 2/7] perf: arm64: Add test to check userspace access to hardware counters Raphael Gault
2019-06-11 12:53 ` [PATCH 3/7] perf: arm64: Use rseq to test userspace access to pmu counters Raphael Gault
2019-06-11 14:33   ` Arnaldo Carvalho de Melo
2019-06-11 16:57     ` Mark Rutland
2019-06-11 19:33       ` Mathieu Desnoyers
2019-06-13  7:10         ` Raphael Gault
2019-06-13  8:27           ` Mathieu Desnoyers
2019-06-13 11:02         ` Raphael Gault
2019-06-25 13:20         ` Raphael Gault
2019-06-25 13:29       ` Raphael Gault
2019-06-25 13:41         ` Will Deacon [this message]
2019-06-11 12:53 ` [PATCH 4/7] arm64: pmu: Add function implementation to update event index in userpage Raphael Gault
2019-06-11 12:53 ` [PATCH 5/7] arm64: pmu: Add hook to handle pmu-related undefined instructions Raphael Gault
2019-06-11 12:53 ` [PATCH 6/7] arm64: perf: Enable pmu counter direct access for perf event on armv8 Raphael Gault
2019-06-11 12:53 ` [PATCH 7/7] Documentation: arm64: Document PMU counters access from userspace Raphael Gault

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