linux-kernel - [PATCH v4 01/36] x86/bugs: Restructure mds mitigation

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Message-ID: <20250310164023.779191-2-david.kaplan@amd.com>
Date: Mon, 10 Mar 2025 11:39:48 -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: <linux-kernel@...r.kernel.org>, Brendan Jackman <jackmanb@...gle.com>,
	Derek Manwaring <derekmn@...zon.com>
Subject: [PATCH v4 01/36] x86/bugs: Restructure mds mitigation

Restructure mds mitigation selection to use select/update/apply
functions to create consistent vulnerability handling.

Signed-off-by: David Kaplan <david.kaplan@....com>
---
 arch/x86/kernel/cpu/bugs.c | 55 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 53 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 4386aa6c69e1..71da57c4f83b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -34,6 +34,25 @@
 
 #include "cpu.h"
 
+/*
+ * Speculation Vulnerability Handling
+ *
+ * Each vulnerability is handled with the following functions:
+ *   <vuln>_select_mitigation() -- Selects a mitigation to use.  This should
+ *				   take into account all relevant command line
+ *				   options.
+ *   <vuln>_update_mitigation() -- This is called after all vulnerabilities have
+ *				   selected a mitigation, in case the selection
+ *				   may want to change based on other choices
+ *				   made.  This function is optional.
+ *   <vuln>_apply_mitigation() -- Enable the selected mitigation.
+ *
+ * The compile-time mitigation in all cases should be AUTO.  An explicit
+ * command-line option can override AUTO.  If no such option is
+ * provided, <vuln>_select_mitigation() will override AUTO to the best
+ * mitigation option.
+ */
+
 static void __init spectre_v1_select_mitigation(void);
 static void __init spectre_v2_select_mitigation(void);
 static void __init retbleed_select_mitigation(void);
@@ -41,6 +60,8 @@ static void __init spectre_v2_user_select_mitigation(void);
 static void __init ssb_select_mitigation(void);
 static void __init l1tf_select_mitigation(void);
 static void __init mds_select_mitigation(void);
+static void __init mds_update_mitigation(void);
+static void __init mds_apply_mitigation(void);
 static void __init md_clear_update_mitigation(void);
 static void __init md_clear_select_mitigation(void);
 static void __init taa_select_mitigation(void);
@@ -169,6 +190,7 @@ void __init cpu_select_mitigations(void)
 	spectre_v2_user_select_mitigation();
 	ssb_select_mitigation();
 	l1tf_select_mitigation();
+	mds_select_mitigation();
 	md_clear_select_mitigation();
 	srbds_select_mitigation();
 	l1d_flush_select_mitigation();
@@ -179,6 +201,14 @@ void __init cpu_select_mitigations(void)
 	 */
 	srso_select_mitigation();
 	gds_select_mitigation();
+
+	/*
+	 * After mitigations are selected, some may need to update their
+	 * choices.
+	 */
+	mds_update_mitigation();
+
+	mds_apply_mitigation();
 }
 
 /*
@@ -281,6 +311,9 @@ enum rfds_mitigations {
 static enum rfds_mitigations rfds_mitigation __ro_after_init =
 	IS_ENABLED(CONFIG_MITIGATION_RFDS) ? RFDS_MITIGATION_AUTO : RFDS_MITIGATION_OFF;
 
+/* Set if any of MDS/TAA/MMIO/RFDS are going to enable VERW. */
+static bool verw_mitigation_selected __ro_after_init;
+
 static void __init mds_select_mitigation(void)
 {
 	if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
@@ -291,12 +324,31 @@ static void __init mds_select_mitigation(void)
 	if (mds_mitigation == MDS_MITIGATION_AUTO)
 		mds_mitigation = MDS_MITIGATION_FULL;
 
+	verw_mitigation_selected = true;
+}
+
+static void __init mds_update_mitigation(void)
+{
+	if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off())
+		return;
+
+	/* If TAA, MMIO, or RFDS are being mitigated, MDS gets mitigated too. */
+	if (verw_mitigation_selected)
+		mds_mitigation = MDS_MITIGATION_FULL;
+
 	if (mds_mitigation == MDS_MITIGATION_FULL) {
 		if (!boot_cpu_has(X86_FEATURE_MD_CLEAR))
 			mds_mitigation = MDS_MITIGATION_VMWERV;
+	}
 
-		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+	pr_info("%s\n", mds_strings[mds_mitigation]);
+}
 
+static void __init mds_apply_mitigation(void)
+{
+	if (mds_mitigation == MDS_MITIGATION_FULL ||
+	    mds_mitigation == MDS_MITIGATION_VMWERV) {
+		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
 		if (!boot_cpu_has(X86_BUG_MSBDS_ONLY) &&
 		    (mds_nosmt || cpu_mitigations_auto_nosmt()))
 			cpu_smt_disable(false);
@@ -599,7 +651,6 @@ static void __init md_clear_update_mitigation(void)
 
 static void __init md_clear_select_mitigation(void)
 {
-	mds_select_mitigation();
 	taa_select_mitigation();
 	mmio_select_mitigation();
 	rfds_select_mitigation();
-- 
2.34.1