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

[Ian Rogers]

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

> ---
>  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
>