From: Christian Weisgerber <naddy@mips.inka.de>
Subject: libcrypto: switch aarch64 CPU feature detection to elf_aux_info()
To: tech@openbsd.org
Cc: Brent Cook <bcook@openbsd.org>
Date: Tue, 4 Aug 2026 19:30:11 +0200

The aarch64 runtime environment provides pre-parsed CPU feature
capabilities for free.  We can just grab them with elf_aux_info()
and no longer need to read the AA64ISAR0 register with sysctl or
extract the bit fields.

OK?

Hint for -portable: This builds as-is on FreeBSD, too.

-----------------------------------------------
commit fa0e19296490832c8f496272bd150d14d170fb50
from: Christian Weisgerber <naddy@mips.inka.de>
date: Tue Aug  4 17:10:07 2026 UTC
 
 switch aarch64 CPU feature detection to elf_aux_info()
 
diff b2b8205756684c6db49759a3153d75acd569191f fa0e19296490832c8f496272bd150d14d170fb50
commit - b2b8205756684c6db49759a3153d75acd569191f
commit + fa0e19296490832c8f496272bd150d14d170fb50
blob - b31a50ef7530b47b579bcaae6b551fd22bf5a861
blob + b9e12776a66281cc4e674de3e80f4a0c9991c89d
--- lib/libcrypto/arch/aarch64/crypto_cpu_caps.c
+++ lib/libcrypto/arch/aarch64/crypto_cpu_caps.c
@@ -15,11 +15,8 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
-#include <sys/types.h>
-#include <sys/sysctl.h>
+#include <sys/auxv.h>
 
-#include <machine/cpu.h>
-
 #include <stddef.h>
 #include <stdio.h>
 
@@ -28,70 +25,29 @@
 /* Machine dependent CPU capabilities. */
 uint64_t crypto_cpu_caps_aarch64;
 
-static inline uint64_t
-extract_bits(uint64_t val, int start, int end)
-{
-	return (val >> end) & (1ULL << (1 + start - end)) - 1;
-}
-
-static uint64_t
-parse_isar0(uint64_t isar0)
-{
-	uint64_t caps = 0;
-	uint64_t feature;
-
-	/* AES - bits [7:4] */
-	feature = extract_bits(isar0, 7, 4);
-	if (feature >= 1)
-		caps |= CRYPTO_CPU_CAPS_AARCH64_AES;
-	if (feature >= 2)
-		caps |= CRYPTO_CPU_CAPS_AARCH64_PMULL;
-
-	/* SHA1 - bits [11:8] */
-	feature = extract_bits(isar0, 11, 8);
-	if (feature >= 1)
-		caps |= CRYPTO_CPU_CAPS_AARCH64_SHA1;
-
-	/* SHA2 - bits [15:12] */
-	feature = extract_bits(isar0, 15, 12);
-	if (feature >= 1)
-		caps |= CRYPTO_CPU_CAPS_AARCH64_SHA2;
-	if (feature >= 2)
-		caps |= CRYPTO_CPU_CAPS_AARCH64_SHA512;
-
-	/* SHA3 - bits [35:32] */
-	feature = extract_bits(isar0, 35, 32);
-	if (feature >= 1)
-		caps |= CRYPTO_CPU_CAPS_AARCH64_SHA3;
-
-	return caps;
-}
-
-static int
-read_isar0(uint64_t *isar0)
-{
-	uint64_t isar;
-	int mib[2];
-	size_t len;
-
-	mib[0] = CTL_MACHDEP;
-	mib[1] = CPU_ID_AA64ISAR0;
-	len = sizeof(isar);
-	if (sysctl(mib, 2, &isar, &len, NULL, 0) == -1)
-		return 0;
-
-	*isar0 = isar;
-
-	return 1;
-}
-
 void
 crypto_cpu_caps_init(void)
 {
-	uint64_t isar = 0;
+	unsigned long hwcap;
 
-	if (!read_isar0(&isar))
+	if (elf_aux_info(AT_HWCAP, &hwcap, sizeof(hwcap)) != 0)
 		return;
 
-	crypto_cpu_caps_aarch64 = parse_isar0(isar);
+	if (hwcap & HWCAP_AES)
+		crypto_cpu_caps_aarch64 |= CRYPTO_CPU_CAPS_AARCH64_AES;
+
+	if (hwcap & HWCAP_PMULL)
+		crypto_cpu_caps_aarch64 |= CRYPTO_CPU_CAPS_AARCH64_PMULL;
+
+	if (hwcap & HWCAP_SHA1)
+		crypto_cpu_caps_aarch64 |= CRYPTO_CPU_CAPS_AARCH64_SHA1;
+
+	if (hwcap & HWCAP_SHA2)
+		crypto_cpu_caps_aarch64 |= CRYPTO_CPU_CAPS_AARCH64_SHA2;
+
+	if (hwcap & HWCAP_SHA512)
+		crypto_cpu_caps_aarch64 |= CRYPTO_CPU_CAPS_AARCH64_SHA512;
+
+	if (hwcap & HWCAP_SHA3)
+		crypto_cpu_caps_aarch64 |= CRYPTO_CPU_CAPS_AARCH64_SHA3;
 }

-- 
Christian "naddy" Weisgerber                          naddy@mips.inka.de