ARM: 9274/1: Add hwcap for Speculative Store Bypassing Safe

Speculative Store Bypassing Safe(FEAT_SSBS) is a feature present in
AArch32 state for Armv8 and is represented by ID_PFR2_EL1.SSBS
identification register.

This feature denotes the presence of PSTATE.ssbs bit and hence adding a
hwcap will enable the userspace to check it before trying to set/unset
this PSTATE.

This commit adds the ID feature bit detection, and uses elf_hwcap2
accordingly.

Reviewed-by: Linus Walleij <[e-mail]>
Signed-off-by: Amit Daniel Kachhap <[e-mail]>
Signed-off-by: Russell King (Oracle) <[e-mail]>

arch/arm/include/uapi/asm/hwcap.h

···
 #define HWCAP2_SHA2	(1 << 3)
 #define HWCAP2_CRC32	(1 << 4)
 #define HWCAP2_SB	(1 << 5)
+#define HWCAP2_SSBS	(1 << 6)
 
 #endif /* _UAPI__ASMARM_HWCAP_H */

arch/arm/kernel/setup.c

···
 	int block;
 	u32 isar5;
 	u32 isar6;
+	u32 pfr2;
 
 	if (cpu_architecture() < CPU_ARCH_ARMv7)
 		return;
···
 	block = cpuid_feature_extract_field(isar6, 12);
 	if (block >= 1)
 		elf_hwcap2 |= HWCAP2_SB;
+
+	/* Check for Speculative Store Bypassing control */
+	pfr2 = read_cpuid_ext(CPUID_EXT_PFR2);
+	block = cpuid_feature_extract_field(pfr2, 4);
+	if (block >= 1)
+		elf_hwcap2 |= HWCAP2_SSBS;
 }
 
 static void __init elf_hwcap_fixup(void)
···
 	"sha2",
 	"crc32",
 	"sb",
+	"ssbs",
 	NULL
 };