linux-kernel - [RFC][PATCH 10/17] x86: Remove CPU_IBPB

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Message-ID: <20230809072201.125308282@infradead.org>
Date:   Wed, 09 Aug 2023 09:12:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        David.Kaplan@....com, Andrew.Cooper3@...rix.com,
        jpoimboe@...nel.org, gregkh@...uxfoundation.org
Subject: [RFC][PATCH 10/17] x86: Remove CPU_IBPB_ENTRY

It was a pointless .config knob, it didn't even cause entry_ibpb to be
omitted from the build.

Our Kconfig space is definitely too big to carry pointless ones.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/Kconfig                     |    7 -------
 arch/x86/include/asm/nospec-branch.h |    9 +++------
 arch/x86/kernel/cpu/bugs.c           |   18 +++++-------------
 3 files changed, 8 insertions(+), 26 deletions(-)

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2577,13 +2577,6 @@ config CALL_THUNKS_DEBUG
 	  Only enable this when you are debugging call thunks as this
 	  creates a noticeable runtime overhead. If unsure say N.
 
-config CPU_IBPB_ENTRY
-	bool "Enable IBPB on kernel entry"
-	depends on CPU_SUP_AMD && X86_64
-	default y
-	help
-	  Compile the kernel with support for the retbleed=ibpb mitigation.
-
 config CPU_IBRS_ENTRY
 	bool "Enable IBRS on kernel entry"
 	depends on CPU_SUP_INTEL && X86_64
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -288,8 +288,7 @@
  * where we have a stack but before any RET instruction.
  */
 .macro UNTRAIN_RET
-#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
-    defined(CONFIG_CALL_DEPTH_TRACKING)
+#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CALL_DEPTH_TRACKING)
 	VALIDATE_UNRET_END
 	ALTERNATIVE_3 "",						\
 		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
@@ -299,8 +298,7 @@
 .endm
 
 .macro UNTRAIN_RET_VM
-#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
-    defined(CONFIG_CALL_DEPTH_TRACKING)
+#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CALL_DEPTH_TRACKING)
 	VALIDATE_UNRET_END
 	ALTERNATIVE_3 "",						\
 		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
@@ -310,8 +308,7 @@
 .endm
 
 .macro UNTRAIN_RET_FROM_CALL
-#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
-    defined(CONFIG_CALL_DEPTH_TRACKING)
+#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CALL_DEPTH_TRACKING)
 	VALIDATE_UNRET_END
 	ALTERNATIVE_3 "",						\
 		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -890,16 +890,13 @@ static void __init retbleed_select_mitig
 		if (!boot_cpu_has(X86_FEATURE_IBPB)) {
 			pr_err("WARNING: CPU does not support IBPB.\n");
 			goto do_cmd_auto;
-		} else if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
+		} else {
 			if (boot_cpu_has_bug(X86_BUG_SRSO) && !has_microcode)
 				pr_err("IBPB-extending microcode not applied; SRSO NOT mitigated\n");
 			if (retbleed_cmd == RETBLEED_CMD_IBPB)
 				retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
 			if (retbleed_cmd == RETBLEED_CMD_IBPB_VMEXIT)
 				retbleed_mitigation = RETBLEED_MITIGATION_IBPB_VMEXIT;
-		} else {
-			pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");
-			goto do_cmd_auto;
 		}
 		break;
 
@@ -932,7 +929,7 @@ static void __init retbleed_select_mitig
 						retbleed_mitigation = RETBLEED_MITIGATION_UNRET_SRSO;
 				}
 			}
-			else if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY) && boot_cpu_has(X86_FEATURE_IBPB))
+			else if (boot_cpu_has(X86_FEATURE_IBPB))
 				retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
 		}
 
@@ -2397,14 +2394,9 @@ static void __init srso_select_mitigatio
 		break;
 
 	case SRSO_CMD_IBPB:
-		if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
-			if (has_microcode) {
-				setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
-				srso_mitigation = SRSO_MITIGATION_IBPB;
-			}
-		} else {
-			pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");
-			goto pred_cmd;
+		if (has_microcode) {
+			setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
+			srso_mitigation = SRSO_MITIGATION_IBPB;
 		}
 		break;