arm64/sysreg: Add definitions for architected GCS caps

The architecture defines a format for guarded control stack caps, used
to mark the top of an unused GCS in order to limit the potential for
exploitation via stack switching. Add definitions associated with these.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-8-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/sysreg.h

@@ -1101,6 +1101,26 @@
 /* Initial value for Permission Overlay Extension for EL0 */
 #define POR_EL0_INIT	POE_RXW
 
+/*
+ * Definitions for Guarded Control Stack
+ */
+
+#define GCS_CAP_ADDR_MASK		GENMASK(63, 12)
+#define GCS_CAP_ADDR_SHIFT		12
+#define GCS_CAP_ADDR_WIDTH		52
+#define GCS_CAP_ADDR(x)			FIELD_GET(GCS_CAP_ADDR_MASK, x)
+
+#define GCS_CAP_TOKEN_MASK		GENMASK(11, 0)
+#define GCS_CAP_TOKEN_SHIFT		0
+#define GCS_CAP_TOKEN_WIDTH		12
+#define GCS_CAP_TOKEN(x)		FIELD_GET(GCS_CAP_TOKEN_MASK, x)
+
+#define GCS_CAP_VALID_TOKEN		0x1
+#define GCS_CAP_IN_PROGRESS_TOKEN	0x5
+
+#define GCS_CAP(x)	((((unsigned long)x) & GCS_CAP_ADDR_MASK) | \
+					       GCS_CAP_VALID_TOKEN)
+
 #define ARM64_FEATURE_FIELD_BITS	4
 
 /* Defined for compatibility only, do not add new users. */