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References: <20240312132508.423320-1-james.clark@arm.com>
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From: Ian Rogers <irogers@google.com>
Date: Tue, 12 Mar 2024 12:56:09 -0700
Message-ID: <CAP-5=fWGrkjx1vz+2aQU0A+_B3d=nCtK9WBGiSPbDbUaTXR4pQ@mail.gmail.com>
Subject: Re: [PATCH] perf docs: arm_spe: Clarify more SPE requirements
To: James Clark <james.clark@arm.com>
Cc: linux-perf-users@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>, 
	Ingo Molnar <mingo@redhat.com>, Arnaldo Carvalho de Melo <acme@kernel.org>, Namhyung Kim <namhyung@kernel.org>, 
	Mark Rutland <mark.rutland@arm.com>, 
	Alexander Shishkin <alexander.shishkin@linux.intel.com>, Jiri Olsa <jolsa@kernel.org>, 
	Adrian Hunter <adrian.hunter@intel.com>, linux-kernel@vger.kernel.org
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On Tue, Mar 12, 2024 at 6:25=E2=80=AFAM James Clark <james.clark@arm.com> w=
rote:
>
> 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

> ---
>  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/Docum=
entation/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 bo=
oted with page table isolati
>  (kpti=3Doff). If KPTI needs to be disabled, this will fail with a consol=
e message "profiling buffer
>  inaccessible. Try passing 'kpti=3Doff' 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 ma=
ndatory.
> +
> +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 interrup=
t 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 describe=
d by firmware,
> +   or running on a VM. See 'Kernel Requirements' above.
>
>   - "Arm SPE CONTEXT packets not found in the traces."
>
> --
> 2.34.1
>