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

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Date: Mon, 3 Jun 2024 14:48:58 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: James Clark <james.clark@....com>
Cc: 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,
 mark.rutland@....com
Subject: Re: [PATCH V17 0/9] arm64/perf: Enable branch stack sampling



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.