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SubjectRe: [PATCH] arm64: errata: add detection for AMEVCNTR01 incrementing incorrectly
On Tue, Jun 07, 2022 at 01:53:40PM +0100, Ionela Voinescu wrote:
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 14a8f3d93add..80e0c700cecf 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -881,11 +881,16 @@ static inline bool cpu_has_pan(void)
> #ifdef CONFIG_ARM64_AMU_EXTN
> /* Check whether the cpu supports the Activity Monitors Unit (AMU) */
> extern bool cpu_has_amu_feat(int cpu);
> +extern bool cpu_has_broken_amu_constcnt(void);
> #else
> static inline bool cpu_has_amu_feat(int cpu)
> {
> return false;
> }
> +static inline bool cpu_has_broken_amu_constcnt(void)
> +{
> + return false;
> +}
> #endif
>
> /* Get a cpu that supports the Activity Monitors Unit (AMU) */
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 42ea2bd856c6..b9e4b2bd2c63 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1791,6 +1791,19 @@ int get_cpu_with_amu_feat(void)
> return cpumask_any(&amu_cpus);
> }
>
> +bool cpu_has_broken_amu_constcnt(void)
> +{
> + /* List of CPUs which have broken AMEVCNTR01 (constant counter) */
> + static const struct midr_range cpus[] = {
> +#ifdef CONFIG_ARM64_ERRATUM_2457168
> + MIDR_RANGE(MIDR_CORTEX_A510, 0, 0, 1, 1),
> +#endif
> + {},
> + };
> +
> + return is_midr_in_range(read_cpuid_id(), cpus);
> +}

I'd rather not have this in cpufeature.c as it's not really a feature.
We have some precedent with checking errata in cpufeature.c but IIRC we
did that only to check whether to enable a feature or not in that file
(DBM).

> +
> static void cpu_amu_enable(struct arm64_cpu_capabilities const *cap)
> {
> if (has_cpuid_feature(cap, SCOPE_LOCAL_CPU)) {
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index 9ab78ad826e2..d4b0b0a40515 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -127,7 +127,8 @@ int __init parse_acpi_topology(void)
>
> #ifdef CONFIG_ARM64_AMU_EXTN
> #define read_corecnt() read_sysreg_s(SYS_AMEVCNTR0_CORE_EL0)
> -#define read_constcnt() read_sysreg_s(SYS_AMEVCNTR0_CONST_EL0)
> +#define read_constcnt() (cpu_has_broken_amu_constcnt() ? 0UL : \
> + read_sysreg_s(SYS_AMEVCNTR0_CONST_EL0))

How often is this called? You end up reading the cpuid, comparing the
range on each call. I guess you can't use a cpucap in the arm64_errata[]
array as you want a check per-CPU? Does it matter if we return 0UL on
for all CPUs if one is affected?

--
Catalin

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