Re: [PATCH v2 1/1] arm: perf: Fix ARCH=arm build with GCC

Mark Rutland
2023-12-16T12:44:50Z

On Fri, Dec 15, 2023 at 05:56:48PM +0000, James Clark wrote:
> LLVM ignores everything inside the if statement and doesn't generate
> errors, but GCC doesn't ignore it, resulting in the following error:
>
> drivers/perf/arm_pmuv3.c: In function 'armv8pmu_write_evtype':
> include/linux/bits.h:34:29: error: left shift count >= width of type [-Werror=shift-count-overflow]
> 34 | (((~UL(0)) - (UL(1) << (l)) + 1) & \
>
> Fix it by using GENMASK_ULL which doesn't overflow on arm32 (even though
> the value is never used there).

It would be nice if this could explain the overflow problem, i.e.

| The GENMASK() macro creates masks of type unsigned long, and we use this to
| geenrate the ARMV8_PMU_EVTYPE_TH and ARMV8_PMU_EVTYPE_TC constants. These
| include bits above bit 31, and generating these requires shifting more than the
| size of unsigned long on 32-bit ARM.
|
| Consequently when building for 32-bit arm, GCC warns about their use:
|
| drivers/perf/arm_pmuv3.c: In function 'armv8pmu_write_evtype':
| include/linux/bits.h:34:29: error: left shift count >= width of type [-Werror=shift-count-overflow]
| 34 | (((~UL(0)) - (UL(1) << (l)) + 1) & \
|
| ... though LLVM does not warn as the actual usage is not reachable on 32-bit
| ARM due to `if (IS_ENABLED(...)` checks.
|
| Avoid the warning by using GENMACK_ULL(), which doesn't overflow on 32-bit arm.

> Fixes: 3115ee021bfb ("arm64: perf: Include threshold control fields in PMEVTYPER mask")
> Reported-by: Uwe Kleine-K"onig <u.kleine-koenig@pengutronix.de>
> Closes: https://lore.kernel.org/linux-arm-kernel/20231215120817.h2f3akgv72zhrtqo@pengutronix.de/
> Signed-off-by: James Clark <james.clark@arm.com>

Thanks for this!

Acked-by: Mark Rutland <mark.rutland@arm.com>

Will, Catalin, the broken commit is queued in the arm64 for-next/perf branch
(and merged into for-next/core); is this something we can easily fold in?

Mark.

> ---
> include/linux/perf/arm_pmuv3.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/perf/arm_pmuv3.h b/include/linux/perf/arm_pmuv3.h
> index 0f4d62ef3a9a..46377e134d67 100644
> --- a/include/linux/perf/arm_pmuv3.h
> +++ b/include/linux/perf/arm_pmuv3.h
> @@ -234,8 +234,8 @@
> * PMXEVTYPER: Event selection reg
> */
> #define ARMV8_PMU_EVTYPE_EVENT GENMASK(15, 0) /* Mask for EVENT bits */
> -#define ARMV8_PMU_EVTYPE_TH GENMASK(43, 32)
> -#define ARMV8_PMU_EVTYPE_TC GENMASK(63, 61)
> +#define ARMV8_PMU_EVTYPE_TH GENMASK_ULL(43, 32) /* arm64 only */
> +#define ARMV8_PMU_EVTYPE_TC GENMASK_ULL(63, 61) /* arm64 only */
>
> /*
> * Event filters for PMUv3
> --
> 2.34.1
>