linux-kernel - [PATCH v5 12/16] x86/bugs: Restructure BHI mitigation

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Message-ID: <20250418161721.1855190-13-david.kaplan@amd.com>
Date: Fri, 18 Apr 2025 11:17:17 -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>
Subject: [PATCH v5 12/16] x86/bugs: Restructure BHI mitigation

Restructure BHI mitigation to use select/update/apply functions to create
consistent vulnerability handling.  BHI mitigation was previously selected
from within spectre_v2_select_mitigation() and now is selected from
cpu_select_mitigation() like with all others.

Define new AUTO mitigation for BHI.

Signed-off-by: David Kaplan <david.kaplan@....com>
---
 arch/x86/kernel/cpu/bugs.c | 31 +++++++++++++++++++++++++++----
 1 file changed, 27 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index bb20cfb81015..b7063f58ae88 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -82,6 +82,9 @@ static void __init l1d_flush_select_mitigation(void);
 static void __init srso_select_mitigation(void);
 static void __init gds_select_mitigation(void);
 static void __init gds_apply_mitigation(void);
+static void __init bhi_select_mitigation(void);
+static void __init bhi_update_mitigation(void);
+static void __init bhi_apply_mitigation(void);
 
 /* The base value of the SPEC_CTRL MSR without task-specific bits set */
 u64 x86_spec_ctrl_base;
@@ -208,6 +211,7 @@ void __init cpu_select_mitigations(void)
 	 */
 	srso_select_mitigation();
 	gds_select_mitigation();
+	bhi_select_mitigation();
 
 	/*
 	 * After mitigations are selected, some may need to update their
@@ -223,6 +227,7 @@ void __init cpu_select_mitigations(void)
 	taa_update_mitigation();
 	mmio_update_mitigation();
 	rfds_update_mitigation();
+	bhi_update_mitigation();
 
 	spectre_v1_apply_mitigation();
 	retbleed_apply_mitigation();
@@ -233,6 +238,7 @@ void __init cpu_select_mitigations(void)
 	rfds_apply_mitigation();
 	srbds_apply_mitigation();
 	gds_apply_mitigation();
+	bhi_apply_mitigation();
 }
 
 /*
@@ -1792,12 +1798,13 @@ static bool __init spec_ctrl_bhi_dis(void)
 
 enum bhi_mitigations {
 	BHI_MITIGATION_OFF,
+	BHI_MITIGATION_AUTO,
 	BHI_MITIGATION_ON,
 	BHI_MITIGATION_VMEXIT_ONLY,
 };
 
 static enum bhi_mitigations bhi_mitigation __ro_after_init =
-	IS_ENABLED(CONFIG_MITIGATION_SPECTRE_BHI) ? BHI_MITIGATION_ON : BHI_MITIGATION_OFF;
+	IS_ENABLED(CONFIG_MITIGATION_SPECTRE_BHI) ? BHI_MITIGATION_AUTO : BHI_MITIGATION_OFF;
 
 static int __init spectre_bhi_parse_cmdline(char *str)
 {
@@ -1818,6 +1825,25 @@ static int __init spectre_bhi_parse_cmdline(char *str)
 early_param("spectre_bhi", spectre_bhi_parse_cmdline);
 
 static void __init bhi_select_mitigation(void)
+{
+	if (!boot_cpu_has(X86_BUG_BHI) || cpu_mitigations_off())
+		bhi_mitigation = BHI_MITIGATION_OFF;
+
+	if (bhi_mitigation == BHI_MITIGATION_AUTO)
+		bhi_mitigation = BHI_MITIGATION_ON;
+}
+
+static void __init bhi_update_mitigation(void)
+{
+	if (spectre_v2_cmd == SPECTRE_V2_CMD_NONE)
+		bhi_mitigation = BHI_MITIGATION_OFF;
+
+	if (!boot_cpu_has_bug(X86_BUG_SPECTRE_V2) &&
+	     spectre_v2_cmd == SPECTRE_V2_CMD_AUTO)
+		bhi_mitigation = BHI_MITIGATION_OFF;
+}
+
+static void __init bhi_apply_mitigation(void)
 {
 	if (bhi_mitigation == BHI_MITIGATION_OFF)
 		return;
@@ -1959,9 +1985,6 @@ static void __init spectre_v2_select_mitigation(void)
 	    mode == SPECTRE_V2_RETPOLINE)
 		spec_ctrl_disable_kernel_rrsba();
 
-	if (boot_cpu_has(X86_BUG_BHI))
-		bhi_select_mitigation();
-
 	spectre_v2_enabled = mode;
 	pr_info("%s\n", spectre_v2_strings[mode]);
 
-- 
2.34.1