[PATCH RFC v2 5/9] x86/cpufeatures: Add X86_FEATURE_APX

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

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>
---
Allocating a new feature word for this bit seems excessive at this stage,
given that no other imminent features are quite known.
---
 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 8b7cf13e0acb..51178d4a6308 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -479,6 +479,7 @@
 #define X86_FEATURE_AMD_FAST_CPPC	(21*32 + 5) /* Fast CPPC */
 #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_APX			(21*32 + 8) /* 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 a2fbea0be535..d5e5013e0e9f 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_APX,			X86_FEATURE_XSAVE     },
 	{}
 };
 
diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
index 16f3ca30626a..6c40d5af8479 100644
--- a/arch/x86/kernel/cpu/scattered.c
+++ b/arch/x86/kernel/cpu/scattered.c
@@ -54,6 +54,7 @@ static const struct cpuid_bit cpuid_bits[] = {
 	{ X86_FEATURE_AMD_LBR_V2,		CPUID_EAX,  1, 0x80000022, 0 },
 	{ X86_FEATURE_AMD_LBR_PMC_FREEZE,	CPUID_EAX,  2, 0x80000022, 0 },
 	{ X86_FEATURE_AMD_HETEROGENEOUS_CORES,	CPUID_EAX, 30, 0x80000026, 0 },
+	{ X86_FEATURE_APX,			CPUID_EDX, 21, 0x00000007, 1 },
 	{ 0, 0, 0, 0, 0 }
 };
 
-- 
2.45.2