arm64/sve: Kconfig update and conditional compilation support

This patch adds CONFIG_ARM64_SVE to control building of SVE support
into the kernel, and adds a stub predicate system_supports_sve() to
control conditional compilation and runtime SVE support.

system_supports_sve() just returns false for now: it will be
replaced with a non-trivial implementation in a later patch, once
SVE support is complete enough to be enabled safely.
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/Kconfig

@@ -976,6 +976,17 @@ config ARM64_PMEM
 
 endmenu
 
+config ARM64_SVE
+	bool "ARM Scalable Vector Extension support"
+	default y
+	help
+	  The Scalable Vector Extension (SVE) is an extension to the AArch64
+	  execution state which complements and extends the SIMD functionality
+	  of the base architecture to support much larger vectors and to enable
+	  additional vectorisation opportunities.
+
+	  To enable use of this extension on CPUs that implement it, say Y.
+
 config ARM64_MODULE_CMODEL_LARGE
 	bool
 

arch/arm64/include/asm/cpufeature.h

@@ -262,6 +262,11 @@ static inline bool system_uses_ttbr0_pan(void)
 		!cpus_have_const_cap(ARM64_HAS_PAN);
 }
 
+static inline bool system_supports_sve(void)
+{
+	return false;
+}
+
 #endif /* __ASSEMBLY__ */
 
 #endif