Re: [PATCH 23/38] arm64: cpufeature: Detect PE support for FEAT_NMI

On 9/16/26 10:22, Jinjie Ruan wrote:

在 2026/9/14 18:20, Vladimir Murzin 写道:

quoted

From: Ada Couprie Diaz <redacted>

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 <redacted>
Signed-off-by: Vladimir Murzin <redacted>
Reviewed-by: Jinjie Ruan <redacted>
---
 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 76350b38f0d7..1a4ef4e94997 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 09aa80df602a..c8b1bacdd161 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h

@@ -809,6 +809,16 @@ 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 32102c3912fa..f9fe7a875f85 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),

@@ -2341,6 +2343,51 @@ 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_once("Pseudo NMI enabled, not using architected NMI\n");
+		return false;

Hi, Vladimir,

The user's actual requirement may be to use FEAT_NMI by default when it
is available, and fall back to pseudo-NMI otherwise, since pseudo-NMI
has inherent noise and is only a second choice when there is no hardware
support. Can we design it this way?

Hi Jinjie,

Hmm, currently users have to explicitly opt in to pseudo-NMI (enable
the config option and pass irqchip.gicv3_pseudo_nmi=1 on the command
line), and those users would continue to use pseudo-NMI even if
FEAT_NMI is available.

What you are asking is to prefer FEAT_NMI over pseudo-NMI when it is
available, effectively turning the explicit opt-in to pseudo-NMI into
a hint rather than a strict request, correct?

Cheers
Vladimir

Best regards,
Jinjie

quoted

+	}
+
+	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)
 {

@@ -3233,6 +3280,25 @@ 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 2775ba3359cf..531583822ec2 100644
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps

@@ -52,6 +52,7 @@ HAS_LS64_V
 HAS_LSUI
 HAS_MOPS
 HAS_NESTED_VIRT
+HAS_NMI
 HAS_NV2P1
 HAS_NV3
 HAS_PAN

@@ -82,6 +83,7 @@ MTE
 MTE_ASYMM
 MTE_FAR
 MTE_STORE_ONLY
+NMI
 SME
 SME_FA64
 SME2