Re: [RFC PATCH 28/36] arm64: cpufeature: Detect PE support for FEAT_NMI

[Jinjie Ruan <ruanjinjie@huawei.com>]

On 7/9/2026 8:13 PM, Vladimir Murzin wrote:
> From: Ada Couprie Diaz <ada.coupriediaz@arm.com>
> 
> Use of FEAT_NMI requires that all the PEs in the system and the GIC
> have NMI support. This patch implements the PE part of that detection.
> 
> In order to avoid problematic interactions between real and pseudo NMIs
> we disable the architected feature if the user has enabled pseudo NMIs
> on the command line. If this is done on a system where support for the
> architected feature is detected then a warning is printed during boot in
> order to help users spot what is likely to be a misconfiguration.
> 
> As KVM does not care about the host kernel supporting FEAT_NMI or not
> to allow guests to use it, split the CPU cap in two : ARM64_HAS_NMI is
> the hardware support, ARM64_NMI is the kernel making use of it.
> 
> Co-developed-by: Mark Brown <broonie@kernel.org>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
> Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
> ---
>  arch/arm64/include/asm/cpucaps.h    |  2 +
>  arch/arm64/include/asm/cpufeature.h | 10 +++++
>  arch/arm64/kernel/cpufeature.c      | 68 ++++++++++++++++++++++++++++-
>  arch/arm64/tools/cpucaps            |  2 +
>  4 files changed, 81 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
> index 25c61cda901c..176a63f1cc1b 100644
> --- a/arch/arm64/include/asm/cpucaps.h
> +++ b/arch/arm64/include/asm/cpucaps.h
> @@ -75,6 +75,8 @@ cpucap_is_possible(const unsigned int cap)
>  		return IS_ENABLED(CONFIG_HW_PERF_EVENTS);
>  	case ARM64_HAS_LSUI:
>  		return IS_ENABLED(CONFIG_ARM64_LSUI);
> +	case ARM64_NMI:
> +		return IS_ENABLED(CONFIG_ARM64_NMI);
>  	}
>  
>  	return true;
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 95cd8d4acd26..84b817b29155 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -816,6 +816,16 @@ static __always_inline bool system_uses_irq_prio_masking(void)
>  	return alternative_has_cap_unlikely(ARM64_HAS_GIC_PRIO_MASKING);
>  }
>  
> +static __always_inline bool system_supports_nmi(void)
> +{
> +	return alternative_has_cap_unlikely(ARM64_HAS_NMI);
> +}
> +
> +static __always_inline bool system_uses_nmi(void)
> +{
> +	return alternative_has_cap_unlikely(ARM64_NMI);
> +}
> +
>  static inline bool system_supports_mte(void)
>  {
>  	return alternative_has_cap_unlikely(ARM64_MTE);
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 9a22df0c5120..bb22292ebabc 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -83,13 +83,14 @@
>  #include <asm/cpu_ops.h>
>  #include <asm/fpsimd.h>
>  #include <asm/hwcap.h>
> +#include <asm/hypervisor.h>
>  #include <asm/insn.h>
> +#include <asm/interrupts/common_flags.h>
>  #include <asm/kvm_host.h>
>  #include <asm/mmu.h>
>  #include <asm/mmu_context.h>
>  #include <asm/mpam.h>
>  #include <asm/mte.h>
> -#include <asm/hypervisor.h>
>  #include <asm/processor.h>
>  #include <asm/smp.h>
>  #include <asm/sysreg.h>
> @@ -315,6 +316,7 @@ static const struct arm64_ftr_bits ftr_id_aa64pfr1[] = {
>  	ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_GCS),
>  		       FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_EL1_GCS_SHIFT, 4, 0),
>  	S_ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_EL1_MTE_frac_SHIFT, 4, 0),
> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_EL1_NMI_SHIFT, 4, 0),
>  	ARM64_FTR_BITS(FTR_VISIBLE_IF_IS_ENABLED(CONFIG_ARM64_SME),
>  		       FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_EL1_SME_SHIFT, 4, 0),
>  	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR1_EL1_MPAM_frac_SHIFT, 4, 0),
> @@ -2320,6 +2322,51 @@ static bool has_gic_prio_relaxed_sync(const struct arm64_cpu_capabilities *entry
>  }
>  #endif
>  
> +#ifdef CONFIG_ARM64_NMI
> +static bool can_use_nmi(const struct arm64_cpu_capabilities *entry, int scope)
> +{
> +	/*
> +	 * ARM64_HAS_NMI has a lower index, and is a boot CPU
> +	 * feature, so will be detected earlier.
> +	 */
> +	BUILD_BUG_ON(ARM64_NMI <= ARM64_HAS_NMI);
> +	if (!cpus_have_cap(ARM64_HAS_NMI))
> +		return false;
> +
> +	/*
> +	 * Having both real and pseudo NMIs enabled simultaneously is
> +	 * likely to cause confusion.  Since pseudo NMIs must be
> +	 * enabled with an explicit command line option, if the user
> +	 * has set that option on a system with real NMIs for some
> +	 * reason assume they know what they're doing.
> +	 *
> +	 * ARM64_HAS_GIC_PRIO_MASKING has a lower index, and is a boot CPU
> +	 * feature, so will be detected earlier.
> +	 */
> +	BUILD_BUG_ON(IS_ENABLED(CONFIG_ARM64_PSEUDO_NMI) &&
> +			(ARM64_NMI <= ARM64_HAS_GIC_PRIO_MASKING));
> +	if (cpus_have_cap(ARM64_HAS_GIC_PRIO_MASKING)) {
> +		pr_info("Pseudo NMI enabled, not using architected NMI\n");
> +		return false;

A small suggestion is to change it to pr_info_once. Otherwise, a large
number of print information will be displayed.

Otherwise, LGTM
Reviewed-by: Jinjie Ruan <ruanjinjie@huawei.com>



# dmesg | grep NMI
[    0.000000] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.000000] GICv3: Pseudo-NMIs enabled using relaxed ICC_PMR_EL1
synchronisation
[    0.434430] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.461345] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.475924] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.491364] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.507948] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.523741] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.543870] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.561246] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.579303] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.597354] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.611880] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.630754] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.649572] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.666047] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.681356] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.707386] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.727123] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.744232] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.769575] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.788145] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.811170] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.831739] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.855084] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.876659] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.897121] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.923611] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.943820] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.967216] CPU features: Pseudo NMI enabled, not using architected NMI
[    0.997958] CPU features: Pseudo NMI enabled, not using architected NMI
[    1.027661] CPU features: Pseudo NMI enabled, not using architected NMI
[    1.057660] CPU features: Pseudo NMI enabled, not using architected NMI
[    8.532896] hw perfevents: enabled with armv8_pmuv3 PMU driver, 7
(0,8000003f) counters available, using NMIs
[    8.536298] watchdog: NMI not fully supported


> +	}
> +
> +	return true;
> +}
> +
> +static void nmi_enable(const struct arm64_cpu_capabilities *__unused)
> +{
> +	/*
> +	 * Enable use of NMIs controlled by ALLINT, SPINTMASK should
> +	 * be clear by default but make it explicit that we are using
> +	 * this mode.  Ensure that ALLINT is clear first in order to
> +	 * avoid leaving things masked.
> +	 */
> +	_allint_clear();
> +	sysreg_clear_set(sctlr_el1, SCTLR_EL1_SPINTMASK, SCTLR_EL1_NMI);
> +	isb();
> +}
> +#endif
> +
>  static bool can_trap_icv_dir_el1(const struct arm64_cpu_capabilities *entry,
>  				 int scope)
>  {
> @@ -3198,6 +3245,25 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  		.matches = has_cpuid_feature,
>  		ARM64_CPUID_FIELDS(ID_AA64ISAR3_EL1, LSUI, IMP)
>  	},
> +#endif
> +	{
> +		.type = ARM64_CPUCAP_BOOT_CPU_FEATURE,
> +		.capability = ARM64_HAS_NMI,
> +		.matches = has_cpuid_feature,
> +		ARM64_CPUID_FIELDS(ID_AA64PFR1_EL1, NMI, IMP)
> +	},
> +#ifdef CONFIG_ARM64_NMI
> +	/*
> +	 * Depends on ARM64_HAS_NMI
> +	 * Checks for conflict with pseudo-NMIs, giving them priority.
> +	 */
> +	{
> +		.desc = "Non-maskable Interrupts",
> +		.type = ARM64_CPUCAP_BOOT_CPU_FEATURE,
> +		.capability = ARM64_NMI,
> +		.matches = can_use_nmi,
> +		.cpu_enable = nmi_enable,
> +	},
>  #endif
>  	{},
>  };
> diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
> index 9b85a84f6fd4..2117b3ef0b82 100644
> --- a/arch/arm64/tools/cpucaps
> +++ b/arch/arm64/tools/cpucaps
> @@ -51,6 +51,7 @@ HAS_LS64_V
>  HAS_LSUI
>  HAS_MOPS
>  HAS_NESTED_VIRT
> +HAS_NMI
>  HAS_BBML2_NOABORT
>  HAS_PAN
>  HAS_PMUV3
> @@ -80,6 +81,7 @@ MTE
>  MTE_ASYMM
>  MTE_FAR
>  MTE_STORE_ONLY
> +NMI
>  SME
>  SME_FA64
>  SME2