linux-kernel - [PATCH v2 6/7] x86/its: Remove =stuff dependency on retbleed

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Message-ID: <20250521-eibrs-fix-v2-6-70e2598e932c@linux.intel.com>
Date: Wed, 21 May 2025 19:45:40 -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 6/7] x86/its: Remove =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@...ux.intel.com>
---
 arch/x86/kernel/cpu/bugs.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 59a1f59bf6e2ceda624b6567c5b09ebc524cb9a2..043da9eaf99c5e1f92ae6c56cb7f779bbceae0e0 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1470,13 +1470,8 @@ 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]);
@@ -1488,8 +1483,6 @@ 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))
@@ -1498,6 +1491,11 @@ 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;
 	}
 }
 

-- 
2.34.1