arm64: elf: advertise 8.1 atomic instructions as new hwcap

The ARM v8.1 architecture introduces new atomic instructions to the A64
instruction set for things like cmpxchg, so advertise their availability
to userspace using a hwcap.

Reviewed-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/setup.c

@@ -278,6 +278,19 @@ static void __init setup_processor(void)
 	if (block && !(block & 0x8))
 		elf_hwcap |= HWCAP_CRC32;
 
+	block = (features >> 20) & 0xf;
+	if (!(block & 0x8)) {
+		switch (block) {
+		default:
+		case 2:
+			elf_hwcap |= HWCAP_ATOMICS;
+		case 1:
+			/* RESERVED */
+		case 0:
+			break;
+		}
+	}
+
 #ifdef CONFIG_COMPAT
 	/*
 	 * ID_ISAR5_EL1 carries similar information as above, but pertaining to
@@ -457,6 +470,7 @@ static const char *hwcap_str[] = {
 	"sha1",
 	"sha2",
 	"crc32",
+	"atomics",
 	NULL
 };