Re: [PATCH] perf docs: arm_spe: Clarify more SPE requirements

[James Clark]

On 12/03/2024 19:56, Ian Rogers wrote:
> On Tue, Mar 12, 2024 at 6:25 AM James Clark <james.clark@xxxxxxx> 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@xxxxxxx>
> 
> Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>
> 
> 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
>>
>