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

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Wed, Mar 13, 2024 at 09:41:54AM +0000, James Clark wrote:
> 
> 
> 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 for the review

Thanks, applied to perf-tools-next,

- Arnaldo
 
> >> ---
> >>  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
> >>
> >