Re: [PATCH RFC v2a 5/9] x86/cpufeatures: Add X86_FEATURE_APX

["Chang S. Bae" <chang.seok.bae@intel.com>]

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On 4/11/2025 9:54 AM, Sohil Mehta wrote:
> On 4/11/2025 9:12 AM, Chang S. Bae wrote:
> 
>> +#define X86_FEATURE_APX			(21*32 + 9) /* Advanced #Performance Extensions */
> 
> Is the '#' before 'Performance' intentional? The previous version didn't
> seem to have it.

Oh no — I only meant to update the bit position. My bad!
> I don't think this table follows any specific logic, but recent patches
> have tried grouping by similar features or features with similar
> dependencies.
> 
> I don't have a preference, but should this be inserted closer to other
> XSAVE dependent features?
> 
>      { X86_FEATURE_PKU,                      X86_FEATURE_XSAVE     },
>      { X86_FEATURE_MPX,                      X86_FEATURE_XSAVE     },
>      { X86_FEATURE_XGETBV1,                  X86_FEATURE_XSAVE     },
> +   { X86_FEATURE_APX,                      X86_FEATURE_XSAVE     },
>      { X86_FEATURE_CMOV,                     X86_FEATURE_FXSR      },

> There is a comment on top of that table that says:
> "Please keep the leaf sorted by cpuid_bit.level for faster search."
> 
> Based on that, APX should be inserted here:
> 
>   { X86_FEATURE_INTEL_PPIN,               CPUID_EBX,  0, 0x00000007, 1 },
> +{ X86_FEATURE_APX,                      CPUID_EDX, 21, 0x00000007, 1 },
>   { X86_FEATURE_RRSBA_CTRL,               CPUID_EDX,  2, 0x00000007, 2 },Yeah, both suggestions make sense. Thanks for pointing them out!

I've attached the patch revision.

Thanks,
Chang



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From a3a68edf35c43162a57dad80ea2eecc9c1101246 Mon Sep 17 00:00:00 2001
From: "Chang S. Bae" <chang.seok.bae@intel.com>
Date: Thu, 13 Feb 2025 23:00:28 -0800
Subject: [PATCH RFC v2b 5/9] x86/cpufeatures: Add X86_FEATURE_APX

Intel Advanced Performance Extensions (APX) introduce a new set of
general-purpose registers, managed as an extended state component via the
xstate management facility.

Before enabling this new xstate, define a feature flag to clarify the
dependency in xsave_cpuid_features[]. APX is enumerated under CPUID level
7 with EDX=1. Since this CPUID leaf is not yet allocated, place the flag
in a scattered feature word.

While this feature is intended only for userspace, exposing it via
/proc/cpuinfo is unnecessary. Instead, the existing arch_prctl(2)
mechanism with the ARCH_GET_XCOMP_SUPP option can be used to query the
feature availability.

Finally, clarify that APX depends on XSAVE.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
---
RFC-V2a -> RFC-V2b: Fix typo and organize APX entries orderly (Sohil)
RFC-V2  -> RFC-V2a: Rebased onto v6.15-rc1; resolve conflict with commit
    968e9bc4cef8 ("x86: move ZMM exclusion list into CPU feature flag")
---
 arch/x86/include/asm/cpufeatures.h | 1 +
 arch/x86/kernel/cpu/cpuid-deps.c   | 1 +
 arch/x86/kernel/cpu/scattered.c    | 1 +
 3 files changed, 3 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 6c2c152d8a67..eb73f3f0ec70 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -481,6 +481,7 @@
 #define X86_FEATURE_AMD_HETEROGENEOUS_CORES (21*32 + 6) /* Heterogeneous Core Topology */
 #define X86_FEATURE_AMD_WORKLOAD_CLASS	(21*32 + 7) /* Workload Classification */
 #define X86_FEATURE_PREFER_YMM		(21*32 + 8) /* Avoid ZMM registers due to downclocking */
+#define X86_FEATURE_APX			(21*32 + 9) /* Advanced Performance Extensions */
 
 /*
  * BUG word(s)
diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index 94c062cddfa4..46efcbd6afa4 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -28,6 +28,7 @@ static const struct cpuid_dep cpuid_deps[] = {
 	{ X86_FEATURE_PKU,			X86_FEATURE_XSAVE     },
 	{ X86_FEATURE_MPX,			X86_FEATURE_XSAVE     },
 	{ X86_FEATURE_XGETBV1,			X86_FEATURE_XSAVE     },
+	{ X86_FEATURE_APX,			X86_FEATURE_XSAVE     },
 	{ X86_FEATURE_CMOV,			X86_FEATURE_FXSR      },
 	{ X86_FEATURE_MMX,			X86_FEATURE_FXSR      },
 	{ X86_FEATURE_MMXEXT,			X86_FEATURE_MMX       },
diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
index 16f3ca30626a..ffb80b5ad97f 100644
--- a/arch/x86/kernel/cpu/scattered.c
+++ b/arch/x86/kernel/cpu/scattered.c
@@ -27,6 +27,7 @@ static const struct cpuid_bit cpuid_bits[] = {
 	{ X86_FEATURE_APERFMPERF,		CPUID_ECX,  0, 0x00000006, 0 },
 	{ X86_FEATURE_EPB,			CPUID_ECX,  3, 0x00000006, 0 },
 	{ X86_FEATURE_INTEL_PPIN,		CPUID_EBX,  0, 0x00000007, 1 },
+	{ X86_FEATURE_APX,			CPUID_EDX, 21, 0x00000007, 1 },
 	{ X86_FEATURE_RRSBA_CTRL,		CPUID_EDX,  2, 0x00000007, 2 },
 	{ X86_FEATURE_BHI_CTRL,			CPUID_EDX,  4, 0x00000007, 2 },
 	{ X86_FEATURE_CQM_LLC,			CPUID_EDX,  1, 0x0000000f, 0 },
-- 
2.45.2