Re: [PATCH v22 0/5] arm64/perf: Enable branch stack sampling

[James Clark <james.clark@linaro.org>]

On 20/05/2025 11:27 pm, 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.
> 
> 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-v22
> 
> Changes in v22:
>   - 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@kernel.org>
> ---
> ---
> Anshuman Khandual (4):
>        arm64/sysreg: Add BRBE registers and fields
>        arm64: el2_setup.h: Make __init_el2_fgt labels consistent, again
>        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   |  81 +++-
>   arch/arm64/include/asm/kvm_host.h    |   2 +
>   arch/arm64/include/asm/sysreg.h      |  17 +-
>   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              | 802 +++++++++++++++++++++++++++++++++++
>   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, 1276 insertions(+), 25 deletions(-)
> ---
> base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
> change-id: 20250129-arm-brbe-v19-24d5d9e5e623
> 
> Best regards,

Tested-by: James Clark <james.clark@linaro.org>