linux-kernel - [PATCH v2 11/11] x86/cpu: Use fancy alternatives to get rid of entry_untrain

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Message-ID: <20230814121149.243457126@infradead.org>
Date:   Mon, 14 Aug 2023 13:44:37 +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,
        nik.borisov@...e.com
Subject: [PATCH v2 11/11] x86/cpu: Use fancy alternatives to get rid of entry_untrain_ret()

Use the new nested alternatives to create what is effectively
ALTERNATIVE_5 and merge the dummy entry_untrain_ret stub into
UNTRAIN_RET properly.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/include/asm/nospec-branch.h |   33 ++++++++++++++++++---------------
 arch/x86/kernel/cpu/bugs.c           |    1 -
 arch/x86/lib/retpoline.S             |    7 -------
 3 files changed, 18 insertions(+), 23 deletions(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -272,11 +272,15 @@
 .endm
 
 #ifdef CONFIG_CPU_UNRET_ENTRY
-#define CALL_UNTRAIN_RET	"call entry_untrain_ret"
+#define ALT_UNRET(old)	\
+	__ALTERNATIVE(__ALTERNATIVE(__ALTERNATIVE(old, call retbleed_untrain_ret, X86_FEATURE_UNRET), \
+				    call srso_untrain_ret, X86_FEATURE_SRSO), \
+		      call srso_alias_untrain_ret, X86_FEATURE_SRSO_ALIAS)
 #else
-#define CALL_UNTRAIN_RET	""
+#define ALT_UNRET(old)	old
 #endif
 
+
 /*
  * Mitigate RETBleed for AMD/Hygon Zen uarch. Requires KERNEL CR3 because the
  * return thunk isn't mapped into the userspace tables (then again, AMD
@@ -292,10 +296,10 @@
 #if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
 	defined(CONFIG_CALL_DEPTH_TRACKING) || defined(CONFIG_CPU_SRSO)
 	VALIDATE_UNRET_END
-	ALTERNATIVE_3 "",						\
-		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
-		      "call entry_ibpb", X86_FEATURE_ENTRY_IBPB,	\
-		      __stringify(RESET_CALL_DEPTH), X86_FEATURE_CALL_DEPTH
+
+	__ALTERNATIVE(__ALTERNATIVE(ALT_UNRET(;),
+				    call entry_ibpb, X86_FEATURE_ENTRY_IBPB),
+		      RESET_CALL_DEPTH, X86_FEATURE_CALL_DEPTH)
 #endif
 .endm
 
@@ -303,10 +307,10 @@
 #if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
 	defined(CONFIG_CALL_DEPTH_TRACKING) || defined(CONFIG_CPU_SRSO)
 	VALIDATE_UNRET_END
-	ALTERNATIVE_3 "",						\
-		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
-		      "call entry_ibpb", X86_FEATURE_IBPB_ON_VMEXIT,	\
-		      __stringify(RESET_CALL_DEPTH), X86_FEATURE_CALL_DEPTH
+
+	__ALTERNATIVE(__ALTERNATIVE(ALT_UNRET(;),
+				    call entry_ibpb, X86_FEATURE_IBPB_ON_VMEXIT),
+		      RESET_CALL_DEPTH, X86_FEATURE_CALL_DEPTH)
 #endif
 .endm
 
@@ -314,10 +318,10 @@
 #if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
 	defined(CONFIG_CALL_DEPTH_TRACKING)
 	VALIDATE_UNRET_END
-	ALTERNATIVE_3 "",						\
-		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
-		      "call entry_ibpb", X86_FEATURE_ENTRY_IBPB,	\
-		      __stringify(RESET_CALL_DEPTH_FROM_CALL), X86_FEATURE_CALL_DEPTH
+
+	__ALTERNATIVE(__ALTERNATIVE(ALT_UNRET(;),
+				    call entry_ibpb, X86_FEATURE_ENTRY_IBPB),
+		      RESET_CALL_DEPTH_FROM_CALL, X86_FEATURE_CALL_DEPTH)
 #endif
 .endm
 
@@ -352,7 +356,6 @@ extern void retbleed_untrain_ret(void);
 extern void srso_untrain_ret(void);
 extern void srso_alias_untrain_ret(void);
 
-extern void entry_untrain_ret(void);
 extern void entry_ibpb(void);
 
 extern void (*x86_return_thunk)(void);
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2450,7 +2450,6 @@ static void __init srso_select_mitigatio
 			 * like ftrace, static_call, etc.
 			 */
 			setup_force_cpu_cap(X86_FEATURE_RETHUNK);
-			setup_force_cpu_cap(X86_FEATURE_UNRET);
 
 			if (boot_cpu_data.x86 == 0x19) {
 				setup_force_cpu_cap(X86_FEATURE_SRSO_ALIAS);
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -277,13 +277,6 @@ SYM_CODE_START(srso_return_thunk)
 	ud2
 SYM_CODE_END(srso_return_thunk)
 
-SYM_FUNC_START(entry_untrain_ret)
-	ALTERNATIVE_2 "jmp retbleed_untrain_ret", \
-		      "jmp srso_untrain_ret", X86_FEATURE_SRSO, \
-		      "jmp srso_alias_untrain_ret", X86_FEATURE_SRSO_ALIAS
-SYM_FUNC_END(entry_untrain_ret)
-__EXPORT_THUNK(entry_untrain_ret)
-
 SYM_CODE_START(__x86_return_thunk)
 	UNWIND_HINT_FUNC
 	ANNOTATE_NOENDBR