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Date: Mon, 3 Jun 2024 14:48:58 +0530
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Subject: Re: [PATCH V17 0/9] arm64/perf: Enable branch stack sampling
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To: James Clark <james.clark@arm.com>
Cc: Mark Brown <broonie@kernel.org>, Rob Herring <robh@kernel.org>,
 Marc Zyngier <maz@kernel.org>, Suzuki Poulose <suzuki.poulose@arm.com>,
 Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
 Arnaldo Carvalho de Melo <acme@kernel.org>,
 linux-perf-users@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
 linux-kernel@vger.kernel.org, will@kernel.org, catalin.marinas@arm.com,
 mark.rutland@arm.com
References: <20240405024639.1179064-1-anshuman.khandual@arm.com>
 <5f8fdfd2-a4f9-4fde-ad24-3b76231e61c8@arm.com>
From: Anshuman Khandual <anshuman.khandual@arm.com>
In-Reply-To: <5f8fdfd2-a4f9-4fde-ad24-3b76231e61c8@arm.com>
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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.