linux-kernel - [RFC][PATCH 17/17] x86/cpu: Use fancy alternatives to get rid of entry_untrain

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Date:   Wed, 09 Aug 2023 09:12:35 +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 17/17] 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/lib/retpoline.S             |    7 -------
 2 files changed, 18 insertions(+), 22 deletions(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -271,11 +271,15 @@
 .endm
 
 #ifdef CONFIG_CPU_UNRET_ENTRY
-#define CALL_UNTRAIN_RET	"call entry_untrain_ret"
+#define ALT_UNRET(old)	\
+	__ALTERNATIVE(__ALTERNATIVE(__ALTERNATIVE(old, call zen_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
@@ -290,30 +294,30 @@
 .macro UNTRAIN_RET
 #if defined(CONFIG_CPU_UNRET_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), X86_FEATURE_CALL_DEPTH
+
+	__ALTERNATIVE(__ALTERNATIVE(ALT_UNRET(;),
+				    call entry_ibpb, X86_FEATURE_ENTRY_IBPB),
+		      RESET_CALL_DEPTH, X86_FEATURE_CALL_DEPTH)
 #endif
 .endm
 
 .macro UNTRAIN_RET_VM
 #if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CALL_DEPTH_TRACKING)
 	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
 
 .macro UNTRAIN_RET_FROM_CALL
 #if defined(CONFIG_CPU_UNRET_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
 
@@ -348,7 +352,6 @@ extern void zen_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/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -263,13 +263,6 @@ SYM_CODE_END(srso_safe_ret)
 SYM_FUNC_END(srso_untrain_ret)
 __EXPORT_THUNK(srso_untrain_ret)
 
-SYM_FUNC_START(entry_untrain_ret)
-	ALTERNATIVE_2 "jmp zen_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)
-
 /*
  * Both these do an unbalanced CALL to mess up the RSB, terminate with UD2
  * to indicate noreturn.