Re: [PATCH v3 1/7] arm64: add support for the AMU extension v1

[Suzuki Kuruppassery Poulose <suzuki.poulose@arm.com>]

Hi Ionela,

On 11/02/2020 18:45, Ionela Voinescu wrote:
> The activity monitors extension is an optional extension introduced
> by the ARMv8.4 CPU architecture. This implements basic support for
> version 1 of the activity monitors architecture, AMUv1.
> 
> This support includes:
> - Extension detection on each CPU (boot, secondary, hotplugged)
> - Register interface for AMU aarch64 registers
> - disable_amu kernel parameter to disable detection/counter access
>    at runtime
> 
> Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> ---
>   .../admin-guide/kernel-parameters.txt         | 10 ++
>   arch/arm64/Kconfig                            | 31 ++++++
>   arch/arm64/include/asm/cpucaps.h              |  3 +-
>   arch/arm64/include/asm/cpufeature.h           |  5 +
>   arch/arm64/include/asm/sysreg.h               | 38 ++++++++
>   arch/arm64/kernel/cpufeature.c                | 97 +++++++++++++++++++
>   6 files changed, 183 insertions(+), 1 deletion(-)
> 
...

> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 04cf64e9f0c9..029a473ad273 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -156,6 +156,7 @@ static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = {
>   	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_CSV3_SHIFT, 4, 0),
>   	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_CSV2_SHIFT, 4, 0),
>   	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_DIT_SHIFT, 4, 0),
> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_AMU_SHIFT, 4, 0),
>   	ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_SVE),
>   				   FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_SVE_SHIFT, 4, 0),
>   	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_RAS_SHIFT, 4, 0),
> @@ -1150,6 +1151,84 @@ static bool has_hw_dbm(const struct arm64_cpu_capabilities *cap,
>   
>   #endif
>   
> +#ifdef CONFIG_ARM64_AMU_EXTN
> +
> +/*
> + * The "amu_cpus" cpumask only signals that the CPU implementation for the
> + * flagged CPUs supports the Activity Monitors Unit (AMU) but does not provide
> + * information regarding all the events that it supports. When a CPU bit is
> + * set in the cpumask, the user of this feature can only rely on the presence
> + * of the 4 fixed counters for that CPU. But this does not guarantee that the
> + * counters are enabled or access to these counters is enabled by code
> + * executed at higher exception levels (firmware).
> + */
> +static cpumask_var_t amu_cpus;
> +
> +bool cpu_has_amu_feat(int cpu)
> +{
> +	if (cpumask_available(amu_cpus))
> +		return cpumask_test_cpu(cpu, amu_cpus);
> +
> +	return false;
> +}
> +
> +static void cpu_amu_enable(struct arm64_cpu_capabilities const *cap)
> +{
> +	if (has_cpuid_feature(cap, SCOPE_LOCAL_CPU)) {
> +		pr_info("detected CPU%d: Activity Monitors Unit (AMU)\n",
> +			smp_processor_id());
> +		cpumask_set_cpu(smp_processor_id(), amu_cpus);
> +	}
> +}
> +
> +/*
> + * For known broken firmware, a kernel parameter ("disable_amu") is provided
> + * to ensure access to AMU counter registers is not attempted. By default,
> + * the feature is enabled, but disable_amu can both be used to disable or
> + * enable the capability at runtime in case the default changes in the future.
> + *
> + * To be noted that for security considerations, this does not bypass the
> + * setting of AMUSERENR_EL0 to trap accesses from EL0 (userspace) to EL1
> + * (kernel). Therefore, firmware should still ensure accesses to AMU registers
> + * are not trapped in EL2/EL3.
> + */
> +static bool disable_amu;
> +
> +static int __init set_disable_amu(char *str)
> +{
> +	int value = 0;
> +
> +	disable_amu = get_option(&str, &value) ? !!value : true;

minor nit: You could simply use strtobool(str) here, which accepts:

disable_amu= [0/1/on/off/y/n]


> +
> +	return 0;
> +}
> +early_param("disable_amu", set_disable_amu);
> +
> +static bool has_amu(const struct arm64_cpu_capabilities *cap,
> +		       int __unused)
> +{
> +	/*
> +	 * The AMU extension is a non-conflicting feature: the kernel can
> +	 * safely run a mix of CPUs with and without support for the
> +	 * activity monitors extension. Therefore, if not disabled through
> +	 * the kernel command line early parameter, enable the capability
> +	 * to allow any late CPU to use the feature.
> +	 *
> +	 * With this feature enabled, the cpu_enable function will be called
> +	 * for all CPUs that match the criteria, including secondary and
> +	 * hotplugged, marking this feature as present on that respective CPU.
> +	 * The enable function will also print a detection message.
> +	 */
> +
> +	if (!disable_amu && !zalloc_cpumask_var(&amu_cpus, GFP_KERNEL)) {

This looks problematic. Don't we end up in allocating the memory during
"each CPU" check and thus leaking memory ? Do we really need to allocate
this dynamically ?

> +		pr_err("Activity Monitors Unit (AMU): fail to allocate memory");
> +		disable_amu = true;
> +	}
> +
> +	return !disable_amu;
> +}
> +#endif
> +
>   #ifdef CONFIG_ARM64_VHE
>   static bool runs_at_el2(const struct arm64_cpu_capabilities *entry, int __unused)
>   {
> @@ -1419,6 +1498,24 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>   		.cpu_enable = cpu_clear_disr,
>   	},
>   #endif /* CONFIG_ARM64_RAS_EXTN */
> +#ifdef CONFIG_ARM64_AMU_EXTN
> +	{
> +		/*
> +		 * The feature is enabled by default if CONFIG_ARM64_AMU_EXTN=y.
> +		 * Therefore, don't provide .desc as we don't want the detection
> +		 * message to be shown until at least one CPU is detected to
> +		 * support the feature.
> +		 */
> +		.capability = ARM64_HAS_AMU_EXTN,
> +		.type = ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE,
> +		.matches = has_amu,
> +		.sys_reg = SYS_ID_AA64PFR0_EL1,
> +		.sign = FTR_UNSIGNED,
> +		.field_pos = ID_AA64PFR0_AMU_SHIFT,
> +		.min_field_value = ID_AA64PFR0_AMU,
> +		.cpu_enable = cpu_amu_enable,
> +	},
> +#endif /* CONFIG_ARM64_AMU_EXTN */
>   	{
>   		.desc = "Data cache clean to the PoU not required for I/D coherence",
>   		.capability = ARM64_HAS_CACHE_IDC,
> 


Thanks
Suzuki