arm64: ssbd: Add global mitigation state accessor

commit c32e1736 upstream.

We're about to need the mitigation state in various parts of the
kernel in order to do the right thing for userspace and guests.

Let's expose an accessor that will let other subsystems know
about the state.
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm64/include/asm/cpufeature.h

@@ -268,6 +268,16 @@ static inline bool system_uses_ttbr0_pan(void)
 #define ARM64_SSBD_FORCE_ENABLE		2
 #define ARM64_SSBD_MITIGATED		3
 
+static inline int arm64_get_ssbd_state(void)
+{
+#ifdef CONFIG_ARM64_SSBD
+	extern int ssbd_state;
+	return ssbd_state;
+#else
+	return ARM64_SSBD_UNKNOWN;
+#endif
+}
+
 #endif /* __ASSEMBLY__ */
 
 #endif