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perf docs arm_spe: Clarify more SPE requirements related to KPTI

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The question of exactly when KPTI needs to be disabled comes up a lot
because it doesn't always need to be done. Add the relevant kernel
function and some examples that describe the behavior.

Also describe the interrupt requirement and that no error message will
be printed if this isn't met.

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: James Clark <james.clark@arm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240312132508.423320-1-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
This commit is contained in:
James Clark 2024-03-12 13:25:07 +00:00 committed by Arnaldo Carvalho de Melo
parent a672af9139
commit 36f65f9b7a
1 changed files with 11 additions and 1 deletions
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12
tools/perf/Documentation/perf-arm-spe.txt
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@ -116,6 +116,15 @@ Depending on CPU model, the kernel may need to be booted with page table isolati
(kpti=off). If KPTI needs to be disabled, this will fail with a console message "profiling buffer
inaccessible. Try passing 'kpti=off' on the kernel command line".
For the full criteria that determine whether KPTI needs to be forced off or not, see function
unmap_kernel_at_el0() in the kernel sources. Common cases where it's not required
are on the CPUs in kpti_safe_list, or on Arm v8.5+ where FEAT_E0PD is mandatory.
The SPE interrupt must also be described by the firmware. If the module is loaded and KPTI is
disabled (or isn't required to be disabled) but the SPE PMU still doesn't show in
/sys/bus/event_source/devices/, then it's possible that the SPE interrupt isn't described by
ACPI or DT. In this case no warning will be printed by the driver.
Capturing SPE with perf command-line tools
------------------------------------------
@ -199,7 +208,8 @@ Common errors
- "Cannot find PMU `arm_spe'. Missing kernel support?"
Module not built or loaded, KPTI not disabled (see above), or running on a VM
Module not built or loaded, KPTI not disabled, interrupt not described by firmware,
or running on a VM. See 'Kernel Requirements' above.
- "Arm SPE CONTEXT packets not found in the traces."