x86/bugs: Introduce cdt_possible()

In preparation to allow ITS to also enable stuffing aka Call Depth
Tracking (CDT) independently of retbleed, introduce a helper
cdt_possible().

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250611-eibrs-fix-v4-5-5ff86cac6c61@linux.intel.com

Author: pa1gupta

arch/x86/kernel/cpu/bugs.c

@@ -1119,6 +1119,19 @@ early_param("nospectre_v1", nospectre_v1_cmdline);
 
 enum spectre_v2_mitigation spectre_v2_enabled __ro_after_init = SPECTRE_V2_NONE;
 
+/* Depends on spectre_v2 mitigation selected already */
+static inline bool cdt_possible(enum spectre_v2_mitigation mode)
+{
+	if (!IS_ENABLED(CONFIG_MITIGATION_CALL_DEPTH_TRACKING) ||
+	    !IS_ENABLED(CONFIG_MITIGATION_RETPOLINE))
+		return false;
+
+	if (mode == SPECTRE_V2_RETPOLINE)
+		return true;
+
+	return false;
+}
+
 #undef pr_fmt
 #define pr_fmt(fmt)     "RETBleed: " fmt
 
@@ -1267,7 +1280,7 @@ static void __init retbleed_update_mitigation(void)
 		retbleed_mitigation = RETBLEED_MITIGATION_STUFF;
 
 	if (retbleed_mitigation == RETBLEED_MITIGATION_STUFF &&
-	    spectre_v2_enabled != SPECTRE_V2_RETPOLINE) {
+	    !cdt_possible(spectre_v2_enabled)) {
 		pr_err("WARNING: retbleed=stuff depends on spectre_v2=retpoline\n");
 		retbleed_mitigation = RETBLEED_MITIGATION_NONE;
 	}