Messages in this thread | | | Date | Wed, 13 Mar 2024 09:41:54 +0000 | Subject | Re: [PATCH] perf docs: arm_spe: Clarify more SPE requirements | From | James Clark <> |
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On 12/03/2024 19:56, Ian Rogers wrote: > On Tue, Mar 12, 2024 at 6:25 AM James Clark <james.clark@arm.com> wrote: >> >> 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. >> >> Signed-off-by: James Clark <james.clark@arm.com> > > Reviewed-by: Ian Rogers <irogers@google.com> > > Thanks, > Ian >
Thanks for the review
>> --- >> tools/perf/Documentation/perf-arm-spe.txt | 12 +++++++++++- >> 1 file changed, 11 insertions(+), 1 deletion(-) >> >> diff --git a/tools/perf/Documentation/perf-arm-spe.txt b/tools/perf/Documentation/perf-arm-spe.txt >> index bf03222e9a68..0a3eda482307 100644 >> --- a/tools/perf/Documentation/perf-arm-spe.txt >> +++ b/tools/perf/Documentation/perf-arm-spe.txt >> @@ -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." >> >> -- >> 2.34.1 >> >
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