[RFC][PATCH 09/17] x86: Remove CONFIG_CPU_SRSO

[Peter Zijlstra]

It doesn't do anything useful anymore (it never really did), remove it
and while at it, put all the untrain and return thunk mess under
CPU_UNRET_ENTRY.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/Kconfig                     |    7 -------
 arch/x86/include/asm/nospec-branch.h |    9 ++++-----
 arch/x86/kernel/vmlinux.lds.S        |    9 +++------
 arch/x86/lib/retpoline.S             |   23 +++++++++++++++++------
 4 files changed, 24 insertions(+), 24 deletions(-)

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2593,13 +2593,6 @@ config CPU_IBRS_ENTRY
 	  This mitigates both spectre_v2 and retbleed at great cost to
 	  performance.
 
-config CPU_SRSO
-	bool "Mitigate speculative RAS overflow on AMD"
-	depends on CPU_SUP_AMD && X86_64 && RETHUNK
-	default y
-	help
-	  Enable the SRSO mitigation needed on AMD Zen1-4 machines.
-
 config SLS
 	bool "Mitigate Straight-Line-Speculation"
 	depends on CC_HAS_SLS && X86_64
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -211,8 +211,7 @@
  * eventually turn into it's own annotation.
  */
 .macro VALIDATE_UNRET_END
-#if defined(CONFIG_NOINSTR_VALIDATION) && \
-	(defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_SRSO))
+#if defined(CONFIG_NOINSTR_VALIDATION) && defined(CONFIG_CPU_UNRET_ENTRY)
 	ANNOTATE_RETPOLINE_SAFE
 	nop
 #endif
@@ -290,7 +289,7 @@
  */
 .macro UNTRAIN_RET
 #if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
-	defined(CONFIG_CALL_DEPTH_TRACKING) || defined(CONFIG_CPU_SRSO)
+    defined(CONFIG_CALL_DEPTH_TRACKING)
 	VALIDATE_UNRET_END
 	ALTERNATIVE_3 "",						\
 		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
@@ -301,7 +300,7 @@
 
 .macro UNTRAIN_RET_VM
 #if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
-	defined(CONFIG_CALL_DEPTH_TRACKING) || defined(CONFIG_CPU_SRSO)
+    defined(CONFIG_CALL_DEPTH_TRACKING)
 	VALIDATE_UNRET_END
 	ALTERNATIVE_3 "",						\
 		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
@@ -312,7 +311,7 @@
 
 .macro UNTRAIN_RET_FROM_CALL
 #if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
-	defined(CONFIG_CALL_DEPTH_TRACKING)
+    defined(CONFIG_CALL_DEPTH_TRACKING)
 	VALIDATE_UNRET_END
 	ALTERNATIVE_3 "",						\
 		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -141,13 +141,13 @@ SECTIONS
 		STATIC_CALL_TEXT
 
 		ALIGN_ENTRY_TEXT_BEGIN
-#ifdef CONFIG_CPU_SRSO
+#ifdef CONFIG_CPU_UNRET_ENTRY
 		*(.text.__x86.rethunk_untrain)
 #endif
 
 		ENTRY_TEXT
 
-#ifdef CONFIG_CPU_SRSO
+#ifdef CONFIG_CPU_UNRET_ENTRY
 		/*
 		 * See the comment above srso_untrain_ret_alias()'s
 		 * definition.
@@ -522,12 +522,9 @@ INIT_PER_CPU(irq_stack_backing_store);
            "fixed_percpu_data is not at start of per-cpu area");
 #endif
 
-#ifdef CONFIG_RETHUNK
+#ifdef CONFIG_CPU_UNRET_ENTRY
 . = ASSERT((__x86_return_thunk & 0x3f) == 0, "__x86_return_thunk not cacheline-aligned");
 . = ASSERT((srso_safe_ret & 0x3f) == 0, "srso_safe_ret not cacheline-aligned");
-#endif
-
-#ifdef CONFIG_CPU_SRSO
 /*
  * GNU ld cannot do XOR so do: (A | B) - (A & B) in order to compute the XOR
  * of the two function addresses:
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -132,6 +132,7 @@ SYM_CODE_END(__x86_indirect_jump_thunk_a
  */
 #ifdef CONFIG_RETHUNK
 
+#ifdef CONFIG_CPU_UNRET_ENTRY
 /*
  * srso_untrain_ret_alias() and srso_safe_ret_alias() are placed at
  * special addresses:
@@ -147,7 +148,6 @@ SYM_CODE_END(__x86_indirect_jump_thunk_a
  *
  * As a result, srso_safe_ret_alias() becomes a safe return.
  */
-#ifdef CONFIG_CPU_SRSO
 	.section .text.__x86.rethunk_untrain
 
 SYM_START(srso_untrain_ret_alias, SYM_L_GLOBAL, SYM_A_NONE)
@@ -160,14 +160,11 @@ SYM_FUNC_END(srso_untrain_ret_alias)
 __EXPORT_THUNK(srso_untrain_ret_alias)
 
 	.section .text.__x86.rethunk_safe
-#endif
 
 /* Needs a definition for the __x86_return_thunk alternative below. */
 SYM_START(srso_safe_ret_alias, SYM_L_GLOBAL, SYM_A_NONE)
-#ifdef CONFIG_CPU_SRSO
 	add $8, %_ASM_SP
 	UNWIND_HINT_FUNC
-#endif
 	ANNOTATE_UNRET_SAFE
 	ret
 	int3
@@ -236,8 +233,6 @@ SYM_CODE_END(__x86_return_thunk)
 SYM_FUNC_END(zen_untrain_ret)
 __EXPORT_THUNK(zen_untrain_ret)
 
-EXPORT_SYMBOL(__x86_return_thunk)
-
 /*
  * SRSO untraining sequence for Zen1/2, similar to zen_untrain_ret()
  * above. On kernel entry, srso_untrain_ret() is executed which is a
@@ -293,6 +288,22 @@ SYM_CODE_START(srso_alias_return_thunk)
 	ud2
 SYM_CODE_END(srso_alias_return_thunk)
 
+#else /* CONFIG_CPU_UNRET_ENTRY */
+
+	.section .text.__x86.return_thunk
+
+SYM_CODE_START(__x86_return_thunk)
+	UNWIND_HINT_FUNC
+	ANNOTATE_NOENDBR
+	ANNOTATE_UNRET_SAFE
+	ret
+	int3
+SYM_CODE_END(__x86_return_thunk)
+
+#endif /* CONFIG_CPU_UNRET_ENTRY */
+
+__EXPORT_THUNK(__x86_return_thunk)
+
 #endif /* CONFIG_RETHUNK */
 
 #ifdef CONFIG_CALL_DEPTH_TRACKING