x86/bugs: Remove its=stuff dependency on retbleed

Allow ITS to enable stuffing independent of retbleed. The dependency is only
on retpoline. It is a valid case for retbleed to be mitigated by eIBRS while
ITS deploys stuffing at the same time.

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-v4-6-5ff86cac6c61@linux.intel.com

Author: pa1gupta

arch/x86/kernel/cpu/bugs.c

@@ -1467,13 +1467,8 @@ static void __init its_update_mitigation(void)
 		break;
 	}
 
-	/*
-	 * retbleed_update_mitigation() will try to do stuffing if its=stuff.
-	 * If it can't, such as if spectre_v2!=retpoline, then fall back to
-	 * aligned thunks.
-	 */
 	if (its_mitigation == ITS_MITIGATION_RETPOLINE_STUFF &&
-	    retbleed_mitigation != RETBLEED_MITIGATION_STUFF)
+	    !cdt_possible(spectre_v2_enabled))
 		its_mitigation = ITS_MITIGATION_ALIGNED_THUNKS;
 
 	pr_info("%s\n", its_strings[its_mitigation]);
@@ -1485,8 +1480,6 @@ static void __init its_apply_mitigation(void)
 	case ITS_MITIGATION_OFF:
 	case ITS_MITIGATION_AUTO:
 	case ITS_MITIGATION_VMEXIT_ONLY:
-	/* its=stuff forces retbleed stuffing and is enabled there. */
-	case ITS_MITIGATION_RETPOLINE_STUFF:
 		break;
 	case ITS_MITIGATION_ALIGNED_THUNKS:
 		if (!boot_cpu_has(X86_FEATURE_RETPOLINE))
@@ -1495,6 +1488,11 @@ static void __init its_apply_mitigation(void)
 		setup_force_cpu_cap(X86_FEATURE_RETHUNK);
 		set_return_thunk(its_return_thunk);
 		break;
+	case ITS_MITIGATION_RETPOLINE_STUFF:
+		setup_force_cpu_cap(X86_FEATURE_RETHUNK);
+		setup_force_cpu_cap(X86_FEATURE_CALL_DEPTH);
+		set_return_thunk(call_depth_return_thunk);
+		break;
 	}
 }