linux-kernel - Re: [PATCH V17 0/9] arm64/perf: Enable branch stack sampling

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Date: Mon, 3 Jun 2024 10:39:30 +0100
From: Mark Rutland <mark.rutland@....com>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: James Clark <james.clark@....com>, Mark Brown <broonie@...nel.org>,
	Rob Herring <robh@...nel.org>, Marc Zyngier <maz@...nel.org>,
	Suzuki Poulose <suzuki.poulose@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-perf-users@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	will@...nel.org, catalin.marinas@....com
Subject: Re: [PATCH V17 0/9] arm64/perf: Enable branch stack sampling

On Mon, Jun 03, 2024 at 02:48:58PM +0530, Anshuman Khandual wrote:
> 
> 
> On 5/30/24 15:33, James Clark wrote:
> > 
> > 
> > On 05/04/2024 03:46, Anshuman Khandual wrote:
> >> This series enables perf branch stack sampling support on arm64 platform
> >> via a new arch feature called Branch Record Buffer Extension (BRBE). All
> >> the relevant register definitions could be accessed here.
> >>
> >> https://developer.arm.com/documentation/ddi0601/2021-12/AArch64-Registers
> >>
> >> This series applies on 6.9-rc2.
> >>
> >> Also this series is being hosted below for quick access, review and test.
> >>
> >> https://git.gitlab.arm.com/linux-arm/linux-anshuman.git (brbe_v17)
> >>
> >> There are still some open questions regarding handling multiple perf events
> >> with different privilege branch filters getting on the same PMU, supporting
> >> guest branch stack tracing from the host etc. Finally also looking for some
> >> suggestions regarding supporting BRBE inside the guest. The series has been
> >> re-organized completely as suggested earlier.
> > 
> > For guest support I'm still of this opinion:
> > 
> >   * No support for the host looking into guests (the addresses don't
> >     make sense anyway without also running Perf record in the guest)
> >   * Save and restore the host buffer and registers on guest switch (if
> >     it was ever used by either host or guest)
> >   * Let the guest do whatever it wants with BRBE without any
> >     virtualisation
> > 
> > Merging this with the current PMU virtualistion stuff seems like a lot
> > of work for no use case (host looking into guests). Having said that, it
> > might not even be worth discussing on this patchset apart from "no guest
> > support", and we can do it later to avoid confusion that it's being
> > proposed for this version.
> 
> Agreed, let's just have "no guest support" for now in this proposed series
> without any more additional changes to keep things simpler and separated.
> I will also update the cover letter next time around making this clear.

FWIW, that sounds good to me.

Mark.