[PATCH v5 2/9] x86/cpufeatures: Add the CPUID feature bit for NMI-source reporting

Sohil Mehta
2025-05-07T01:21:25Z

NMI-source reporting is introduced to report the sources of NMIs with
FRED event delivery based on vectors in NMI interrupt messages or the
local APIC. This enables the kernel to avoid the latency incurred by
going over the entire NMI handler list and reduces ambiguity about the
source of an NMI.

Enumerate NMI-source reporting in cpufeatures. Also, since NMI-source
reporting uses the FRED event dispatch framework, make it dependent on
FRED in the CPUID dependency table. This ensures that NMI-source
reporting gets disabled when FRED is disabled.

NMI-source reporting is intended as a kernel feature and does not need
userspace enumeration or configuration. There is no need to expose it to
userspace through /proc/cpuinfo.

Originally-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
---
v5: Add NMI-source to the CPUID dependency table.
Do not expose NMI-source feature through /proc/cpuinfo.
---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/kernel/cpu/cpuid-deps.c | 1 +
2 files changed, 2 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 6c2c152d8a67..2ced1bc64548 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -321,6 +321,7 @@
#define X86_FEATURE_FRED (12*32+17) /* "fred" Flexible Return and Event Delivery */
#define X86_FEATURE_LKGS (12*32+18) /* Load "kernel" (userspace) GS */
#define X86_FEATURE_WRMSRNS (12*32+19) /* Non-serializing WRMSR */
+#define X86_FEATURE_NMI_SOURCE (12*32+20) /* NMI-Source reporting with FRED */
#define X86_FEATURE_AMX_FP16 (12*32+21) /* AMX fp16 Support */
#define X86_FEATURE_AVX_IFMA (12*32+23) /* Support for VPMADD52[H,L]UQ */
#define X86_FEATURE_LAM (12*32+26) /* "lam" Linear Address Masking */
diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index a2fbea0be535..ed0fd35c9290 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -84,6 +84,7 @@ static const struct cpuid_dep cpuid_deps[] = {
{ X86_FEATURE_AMX_TILE, X86_FEATURE_XFD },
{ X86_FEATURE_SHSTK, X86_FEATURE_XSAVES },
{ X86_FEATURE_FRED, X86_FEATURE_LKGS },
+ { X86_FEATURE_NMI_SOURCE, X86_FEATURE_FRED },
{}
};

--
2.43.0