perf docs arm_spe: Clarify more SPE requirements related to KPTI

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.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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."