linux-kernel - Re: [PATCH v23 0/4] arm64/perf: Enable branch stack sampling

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Message-ID: <aGgNVhoqiDcEsyY3@J2N7QTR9R3>
Date: Fri, 4 Jul 2025 18:20:22 +0100
From: Mark Rutland <mark.rutland@....com>
To: "Rob Herring (Arm)" <robh@...nel.org>
Cc: Will Deacon <will@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Joey Gouly <joey.gouly@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	James Clark <james.clark@...aro.org>,
	Anshuman Khandual <anshuman.khandual@....com>,
	Leo Yan <leo.yan@....com>, linux-arm-kernel@...ts.infradead.org,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, kvmarm@...ts.linux.dev,
	Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v23 0/4] arm64/perf: Enable branch stack sampling

Hi Rob,

Thanks again for this.

On Wed, Jun 11, 2025 at 01:01:10PM -0500, Rob Herring (Arm) wrote:
> This series enables perf branch stack sampling support on arm64 via a 
> v9.2 arch feature called Branch Record Buffer Extension (BRBE). Details 
> on BRBE can be found in the Arm ARM[1] chapter D18.
> 
> I've picked up this series from Anshuman. v19 and later versions have 
> been reworked quite a bit by Mark and myself. The bulk of those changes 
> are in patch 5.

Aside from the comments on patch 4, this all looks good to me, and for
the series:

Acked-by: Mark Rutland <mark.rutland@....com>

I assume that Will will pick this up soon.

Mark.

> 
> A git branch is here[2].
> 
> [1] https://developer.arm.com/documentation/ddi0487/latest/
> [2] git://git.kernel.org/pub/scm/linux/kernel/git/robh/linux.git arm/brbe-v23
> 
> v23:
>  - Rebase on v6.16-rc1 dropping already applied el2_setup.h clean-ups
>  - Drop the SYSCALL fixup and convert all BRBE CALL events to SYSCALL
> 
> v22:
>  - https://lore.kernel.org/all/20250520-arm-brbe-v19-v22-0-c1ddde38e7f8@kernel.org/
>  - New patch reworking the labels in el2_setup.h
>  - Move branch stack disabling after armpmu_stop() in armpmu_del()
>  - Fix branch_records_alloc() to work on heterogeneous systems
>  - Make setting .sched_task function ptr conditional on BRBE support
>  - Reword booting.rst section name (s/feature/the/) and move next to
>    other PMU related features instead of in the middle of SME features.
>  - Drop setting SYS_BRBCR_EL1
>  - Drop CONFIG_ARM64_BRBE ifdef
>  - Rework initialization of HFGITR_EL2
> 
> v21:
>  - https://lore.kernel.org/r/20250407-arm-brbe-v19-v21-0-ff187ff6c928@kernel.org
>  - Drop clean-up patches 1-7 already applied
>  - Rebase on v6.15-rc1
> 
> v20:
>  - https://lore.kernel.org/r/20250218-arm-brbe-v19-v20-0-4e9922fc2e8e@kernel.org
>  - Added back some of the arm64 specific exception types. The x86 IRQ 
>    branches also include other exceptions like page faults. On arm64, we 
>    can distinguish the exception types, so we do. Also, to better 
>    align with x86, we convert 'call' branches which are user to kernel 
>    to 'syscall'.
>  - Only enable exceptions and exception returns if recording kernel
>    branches (matching x86)
>  - Drop requiring event and branch privileges to match
>  - Add "branches" caps sysfs attribute like x86
>  - Reword comment about FZP and MDCR_EL2.HPMN interaction
>  - Rework BRBE invalidation to avoid invalidating in interrupt handler
>    when no handled events capture the branch stack (i.e. when there are 
>    multiple users).
>  - Also clear BRBCR_ELx bits in brbe_disable(). This is for KVM nVHE 
>    checks if BRBE is enabled.
>  - Document that MDCR_EL3.SBRBE can be 0b01 also
> 
> v19:
>  - https://lore.kernel.org/all/20250202-arm-brbe-v19-v19-0-1c1300802385@kernel.org/
>  - Drop saving of branch records when task scheduled out (Mark). Make 
>    sched_task() callback actually get called. Enabling requires a call 
>    to perf_sched_cb_inc(). So the saving of branch records never 
>    happened.
>  - Got rid of added armpmu ops. All BRBE support is contained within 
>    pmuv3 code.
>  - Fix freeze on overflow for VHE
>  - The cycle counter doesn't freeze BRBE on overflow, so avoid assigning
>    it when BRBE is enabled.
>  - Drop all the Arm specific exception branches. Not a clear need for
>    them.
>  - Fix handling of branch 'cycles' reading. CC field is
>    mantissa/exponent, not an integer.
>  - Rework s/w filtering to better match h/w filtering
>  - Reject events with disjoint event filter and branch filter or with 
>    exclude_host set
>  - Dropped perf test patch which has been applied for 6.14
>  - Dropped patch "KVM: arm64: Explicitly handle BRBE traps as UNDEFINED"
>    which has been applied for 6.14
> 
> v18:
>  - https://lore.kernel.org/all/20240613061731.3109448-1-anshuman.khandual@arm.com/
> 
> For v1-v17, see the above link. Not going to duplicate it all here...
> 
> Signed-off-by: "Rob Herring (Arm)" <robh@...nel.org>
> ---
> ---
> Anshuman Khandual (3):
>       arm64/sysreg: Add BRBE registers and fields
>       arm64: Handle BRBE booting requirements
>       KVM: arm64: nvhe: Disable branch generation in nVHE guests
> 
> Rob Herring (Arm) (1):
>       perf: arm_pmuv3: Add support for the Branch Record Buffer Extension (BRBE)
> 
>  Documentation/arch/arm64/booting.rst |  21 +
>  arch/arm64/include/asm/el2_setup.h   |  71 +++-
>  arch/arm64/include/asm/kvm_host.h    |   2 +
>  arch/arm64/include/asm/sysreg.h      |  16 +-
>  arch/arm64/kvm/debug.c               |   4 +
>  arch/arm64/kvm/hyp/nvhe/debug-sr.c   |  32 ++
>  arch/arm64/kvm/hyp/nvhe/switch.c     |   2 +-
>  arch/arm64/tools/sysreg              | 132 ++++++
>  drivers/perf/Kconfig                 |  11 +
>  drivers/perf/Makefile                |   1 +
>  drivers/perf/arm_brbe.c              | 798 +++++++++++++++++++++++++++++++++++
>  drivers/perf/arm_brbe.h              |  47 +++
>  drivers/perf/arm_pmu.c               |  16 +-
>  drivers/perf/arm_pmuv3.c             | 125 +++++-
>  include/linux/perf/arm_pmu.h         |   8 +
>  15 files changed, 1265 insertions(+), 21 deletions(-)
> ---
> base-commit: 19272b37aa4f83ca52bdf9c16d5d81bdd1354494
> change-id: 20250129-arm-brbe-v19-24d5d9e5e623
> 
> Best regards,
> -- 
> Rob Herring (Arm) <robh@...nel.org>
>