Re: [PATCH v2 00/14] perf cs-etm: Fix bogus branch samples on exceptions

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

On 23/09/2026 16:21, Leo Yan wrote:
> CoreSight currently models exception entry by changing the preceding
> instruction range into a taken branch. This can give an IRQ the source
> PC of an instruction that already retired and overwrite a real branch
> immediately before the exception.
> 
> For an untaken B.LS followed by an IRQ, perf script currently reports:
> 
>    hw int  4000f8 => ffff800080010c80  b.ls #0x400118
>    ...
>    iret    ffff800080012284 => 4000fc  eret
> 
> The hardware trace supplies 0x4000fc as the preferred return address.
> For an IRQ, this is the architectural resume PC (Arm ARM, R_VBQMV).
> B.LS has retired, and the saved PC identifies the boundary before the
> following MOV. Using this PC as the IRQ source reflects the
> architectural state at exception entry:
> 
>    hw int  4000fc => ffff800080010c80  movz x2, #0x1796
>    ...
>    iret    ffff800080012284 => 4000fc  eret
> 
> The IRQ sample represents the transfer from this architectural execution
> position to the handler. The series synthesizes exception entries from
> exception packets, preserving the preceding branch and its outcome.
> 
> The supporting changes:
> 
> - Let decoders supply sample.ret_addr so later instruction fetching cannot
>    change thread-stack return addresses. Apply this to Intel PT
>    asynchronous samples as well.
> - Prepare packet ISA and instruction-size handling and share sample
>    synthesis helpers. Mark untaken branches and break history when
>    instruction memory is unavailable.
> - Add a thread-stack regression test and four AArch64 CoreSight tests.
>    IRQs and page faults must resume at the entry PC; SVC and emulated MRS
>    must resume four bytes later.
> 
> Based on the AI search and test on my x86 machine, this matches perf's
> Intel PT handling of IRQs. Intel PT records the next instruction's IP in
> the FUP packet, and Perf uses that IP as the interrupt sample's source.
> 
> This series is verified on Orion6 board with "perf test coresight".
> 
> Signed-off-by: Leo Yan <leo.yan@arm.com>
> ---
> Changes in v2:
> 
> - Rework the fix around exception packets to preserve both exception
>    entries and preceding branches.
> - Add explicit return addresses and the Intel PT asynchronous-branch fix.
> - Split out packet/synthesis preparation, record not-taken branches and
>    handle unreadable instruction memory.
> - Add thread-stack regression coverage and four CoreSight tests.
> - Link to v1: https://lore.kernel.org/r/20260713-perf_cs_etm_fix_non_taken-v1-0-4561607fc69f@arm.com
> 
> ---
> Leo Yan (14):
>        perf sample: Allow decoders to supply branch return addresses
>        perf intel-pt: Preserve return addresses for asynchronous branches
>        perf cs-etm: Break branch history when instruction memory is unavailable
>        perf cs-etm: Centralize packet ISA initialization
>        perf cs-etm: Use the recorded instruction size for A32 and A64
>        perf cs-etm: Mark branches that were not taken
>        perf cs-etm: Factor out final instruction sample synthesis
>        perf cs-etm: Centralize branch sample synthesis checks
>        perf cs-etm: Classify exception calls using the exception packet
>        perf cs-etm: Synthesize exception entries separately from branches
>        perf tests: Check CoreSight IRQ entry and exit
>        perf tests: Check CoreSight syscall entry and exit
>        perf tests: Check CoreSight abort entry and exit
>        perf tests: Check CoreSight emulated instruction entry and exit
> 
>   tools/perf/tests/Build                             |   1 +
>   tools/perf/tests/builtin-test.c                    |   3 +
>   .../perf/tests/shell/coresight/abort_entry_exit.sh |  21 ++
>   tools/perf/tests/shell/coresight/irq_entry_exit.sh |  37 ++++
>   .../tests/shell/coresight/syscall_entry_exit.sh    |  21 ++
>   .../perf/tests/shell/coresight/trap_entry_exit.sh  |  24 +++
>   tools/perf/tests/shell/lib/coresight_exception.sh  | 165 +++++++++++++++
>   tools/perf/tests/tests.h                           |   3 +
>   tools/perf/tests/thread-stack.c                    | 106 ++++++++++
>   tools/perf/tests/workloads/Build                   |   4 +
>   tools/perf/tests/workloads/branch_not_taken_loop.c |  33 +++
>   tools/perf/tests/workloads/page_fault_loop.c       |  37 ++++
>   tools/perf/util/cs-etm-decoder/cs-etm-decoder.c    | 106 +++++++---
>   tools/perf/util/cs-etm.c                           | 222 ++++++++++-----------
>   tools/perf/util/cs-etm.h                           |   2 +
>   tools/perf/util/intel-pt.c                         |   7 +
>   tools/perf/util/sample.c                           |   1 +
>   tools/perf/util/sample.h                           |   5 +
>   tools/perf/util/thread-stack.c                     |   5 +-
>   19 files changed, 656 insertions(+), 147 deletions(-)
> ---
> base-commit: edd8a9fe2eca009599e013a29c421c7a6b5ad1b9
> change-id: 20260713-perf_cs_etm_fix_non_taken-5b4d7f73f41e
> 
> Best regards,

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