x86: move ZMM exclusion list into CPU feature flag

Lift zmm_exclusion_list in aesni-intel_glue.c into the x86 CPU setup
code, and add a new x86 CPU feature flag X86_FEATURE_PREFER_YMM that is
set when the CPU is on this list.

This allows other code in arch/x86/, such as the CRC library code, to
apply the same exclusion list when deciding whether to execute 256-bit
or 512-bit optimized functions.

Note that full AVX512 support including ZMM registers is still exposed
to userspace and is still supported for in-kernel use.  This flag just
indicates whether in-kernel code should prefer to use YMM registers.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20250210174540.161705-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

arch/x86/crypto/aesni-intel_glue.c

@@ -1536,26 +1536,6 @@ DEFINE_GCM_ALGS(vaes_avx10_512, FLAG_AVX10_512,
 		AES_GCM_KEY_AVX10_SIZE, 800);
 #endif /* CONFIG_AS_VAES && CONFIG_AS_VPCLMULQDQ */
 
-/*
- * This is a list of CPU models that are known to suffer from downclocking when
- * zmm registers (512-bit vectors) are used.  On these CPUs, the AES mode
- * implementations with zmm registers won't be used by default.  Implementations
- * with ymm registers (256-bit vectors) will be used by default instead.
- */
-static const struct x86_cpu_id zmm_exclusion_list[] = {
-	X86_MATCH_VFM(INTEL_SKYLAKE_X,		0),
-	X86_MATCH_VFM(INTEL_ICELAKE_X,		0),
-	X86_MATCH_VFM(INTEL_ICELAKE_D,		0),
-	X86_MATCH_VFM(INTEL_ICELAKE,		0),
-	X86_MATCH_VFM(INTEL_ICELAKE_L,		0),
-	X86_MATCH_VFM(INTEL_ICELAKE_NNPI,	0),
-	X86_MATCH_VFM(INTEL_TIGERLAKE_L,	0),
-	X86_MATCH_VFM(INTEL_TIGERLAKE,		0),
-	/* Allow Rocket Lake and later, and Sapphire Rapids and later. */
-	/* Also allow AMD CPUs (starting with Zen 4, the first with AVX-512). */
-	{},
-};
-
 static int __init register_avx_algs(void)
 {
 	int err;
@@ -1600,7 +1580,7 @@ static int __init register_avx_algs(void)
 	if (err)
 		return err;
 
-	if (x86_match_cpu(zmm_exclusion_list)) {
+	if (boot_cpu_has(X86_FEATURE_PREFER_YMM)) {
 		int i;
 
 		aes_xts_alg_vaes_avx10_512.base.cra_priority = 1;

arch/x86/include/asm/cpufeatures.h

@@ -483,6 +483,7 @@
 #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_PREFER_YMM		(21*32 + 8) /* Avoid ZMM registers due to downclocking */
 
 /*
  * BUG word(s)

arch/x86/kernel/cpu/intel.c

@@ -521,6 +521,25 @@ static void init_intel_misc_features(struct cpuinfo_x86 *c)
 	wrmsrl(MSR_MISC_FEATURES_ENABLES, msr);
 }
 
+/*
+ * This is a list of Intel CPUs that are known to suffer from downclocking when
+ * ZMM registers (512-bit vectors) are used.  On these CPUs, when the kernel
+ * executes SIMD-optimized code such as cryptography functions or CRCs, it
+ * should prefer 256-bit (YMM) code to 512-bit (ZMM) code.
+ */
+static const struct x86_cpu_id zmm_exclusion_list[] = {
+	X86_MATCH_VFM(INTEL_SKYLAKE_X,		0),
+	X86_MATCH_VFM(INTEL_ICELAKE_X,		0),
+	X86_MATCH_VFM(INTEL_ICELAKE_D,		0),
+	X86_MATCH_VFM(INTEL_ICELAKE,		0),
+	X86_MATCH_VFM(INTEL_ICELAKE_L,		0),
+	X86_MATCH_VFM(INTEL_ICELAKE_NNPI,	0),
+	X86_MATCH_VFM(INTEL_TIGERLAKE_L,	0),
+	X86_MATCH_VFM(INTEL_TIGERLAKE,		0),
+	/* Allow Rocket Lake and later, and Sapphire Rapids and later. */
+	{},
+};
+
 static void init_intel(struct cpuinfo_x86 *c)
 {
 	early_init_intel(c);
@@ -601,6 +620,9 @@ static void init_intel(struct cpuinfo_x86 *c)
 	}
 #endif
 
+	if (x86_match_cpu(zmm_exclusion_list))
+		set_cpu_cap(c, X86_FEATURE_PREFER_YMM);
+
 	/* Work around errata */
 	srat_detect_node(c);