linux-kernel - [RFC PATCH 19/56] x86/bugs: Define bugs_smt

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Message-ID: <20251013143444.3999-20-david.kaplan@amd.com>
Date: Mon, 13 Oct 2025 09:34:07 -0500
From: David Kaplan <david.kaplan@....com>
To: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Ingo Molnar
	<mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
	<x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>
CC: Alexander Graf <graf@...zon.com>, Boris Ostrovsky
	<boris.ostrovsky@...cle.com>, <linux-kernel@...r.kernel.org>
Subject: [RFC PATCH 19/56] x86/bugs: Define bugs_smt_disable()

bugs_smt_disable() is a wrapper around cpu_smt_disable() which can only be
called during boot.  Print a warning if any mitigations try to disable SMT
at runtime.

bugs_smt_disable() is marked __ref because cpu_smt_disable() is an __init
function, but it will never be called after init code is free'd.

Signed-off-by: David Kaplan <david.kaplan@....com>
---
 arch/x86/kernel/cpu/bugs.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index fcb1337026f1..6cd7198f7dca 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -455,6 +455,14 @@ static bool __init should_mitigate_vuln(unsigned int bug)
 	}
 }
 
+static void __ref bugs_smt_disable(bool enable)
+{
+	if (system_state == SYSTEM_BOOTING)
+		cpu_smt_disable(enable);
+	else
+		pr_warn("Unable to disable SMT after system boot!\n");
+}
+
 /* Default mitigation for MDS-affected CPUs */
 static enum mds_mitigations mds_mitigation __ro_after_init =
 	IS_ENABLED(CONFIG_MITIGATION_MDS) ? MDS_MITIGATION_AUTO : MDS_MITIGATION_OFF;
@@ -550,7 +558,7 @@ static void __init mds_apply_mitigation(void)
 		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
 		if (!boot_cpu_has(X86_BUG_MSBDS_ONLY) &&
 		    (mds_nosmt || smt_mitigations == SMT_MITIGATIONS_ON))
-			cpu_smt_disable(false);
+			bugs_smt_disable(false);
 	}
 }
 
@@ -671,7 +679,7 @@ static void __init taa_apply_mitigation(void)
 		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
 
 		if (taa_nosmt || smt_mitigations == SMT_MITIGATIONS_ON)
-			cpu_smt_disable(false);
+			bugs_smt_disable(false);
 	}
 }
 
@@ -784,7 +792,7 @@ static void __init mmio_apply_mitigation(void)
 		static_branch_enable(&cpu_buf_idle_clear);
 
 	if (mmio_nosmt || smt_mitigations == SMT_MITIGATIONS_ON)
-		cpu_smt_disable(false);
+		bugs_smt_disable(false);
 }
 
 #ifdef CONFIG_DYNAMIC_MITIGATIONS
@@ -1592,7 +1600,7 @@ static void __init retbleed_apply_mitigation(void)
 
 	if (mitigate_smt && !boot_cpu_has(X86_FEATURE_STIBP) &&
 	    (retbleed_nosmt || smt_mitigations == SMT_MITIGATIONS_ON))
-		cpu_smt_disable(false);
+		bugs_smt_disable(false);
 }
 
 #ifdef CONFIG_DYNAMIC_MITIGATIONS
@@ -3109,10 +3117,10 @@ static void __init l1tf_apply_mitigation(void)
 		break;
 	case L1TF_MITIGATION_FLUSH_NOSMT:
 	case L1TF_MITIGATION_FULL:
-		cpu_smt_disable(false);
+		bugs_smt_disable(false);
 		break;
 	case L1TF_MITIGATION_FULL_FORCE:
-		cpu_smt_disable(true);
+		bugs_smt_disable(true);
 		break;
 	}
 
-- 
2.34.1