linux-kernel - [PATCH v2 5/7] x86/retbleed: Introduce cdt

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Message-ID: <20250521-eibrs-fix-v2-5-70e2598e932c@linux.intel.com>
Date: Wed, 21 May 2025 19:45:24 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: x86@...nel.org
Cc: David Kaplan <david.kaplan@....com>, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>
Subject: [PATCH v2 5/7] x86/retbleed: Introduce cdt_possible()

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

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
---
 arch/x86/kernel/cpu/bugs.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index fbfc08f5f9f652beb677a1b5310e4322600082ac..59a1f59bf6e2ceda624b6567c5b09ebc524cb9a2 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1123,6 +1123,19 @@ 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
 
@@ -1266,7 +1279,7 @@ 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_AUTO;
 	}

-- 
2.34.1