[xen staging] Revert "x86: guard against straight-line speculation past RET"

[patchbot@xxxxxxx]

commit 9cd905bae87fb06efa59c347ef1e96c5dc6cdb62
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Apr 9 09:50:40 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Apr 9 09:53:53 2021 +0200

    Revert "x86: guard against straight-line speculation past RET"
    
    This reverts commit 71b0b475d801ebeb83a6ba402425135c314fa2df,
    which has no real effect - the most recent version of the patch
    had lost the INT3 insn.
---
 xen/Makefile                    |  5 +----
 xen/arch/x86/indirect-thunk.S   |  3 ---
 xen/include/asm-x86/asm-defns.h | 16 ----------------
 3 files changed, 1 insertion(+), 23 deletions(-)

diff --git a/xen/Makefile b/xen/Makefile
index 3a8fb7ec7e..9f3be7766d 100644
--- a/xen/Makefile
+++ b/xen/Makefile
@@ -145,10 +145,7 @@ t2 = $(call as-insn,$(CC) -I$(BASEDIR)/include,".include 
\"asm-x86/asm-defns.h\"
 # https://bugs.llvm.org/show_bug.cgi?id=36110
 t3 = $(call as-insn,$(CC),".macro FOO;.endm"$(close); asm volatile 
$(open)".macro FOO;.endm",-no-integrated-as)
 
-# Check whether macros can override insn mnemonics in inline assembly.
-t4 = $(call as-insn,$(CC),".macro ret; .error; .endm; .macro retq; .error; 
.endm",-no-integrated-as)
-
-CLANG_FLAGS += $(call or,$(t1),$(t2),$(t3),$(t4))
+CLANG_FLAGS += $(call or,$(t1),$(t2),$(t3))
 endif
 
 CLANG_FLAGS += -Werror=unknown-warning-option
diff --git a/xen/arch/x86/indirect-thunk.S b/xen/arch/x86/indirect-thunk.S
index a090b35dcf..7cc22da0ef 100644
--- a/xen/arch/x86/indirect-thunk.S
+++ b/xen/arch/x86/indirect-thunk.S
@@ -11,9 +11,6 @@
 
 #include <asm/asm_defns.h>
 
-/* Don't transform the "ret" further down. */
-.purgem ret
-
 .macro IND_THUNK_RETPOLINE reg:req
         call 2f
 1:
diff --git a/xen/include/asm-x86/asm-defns.h b/xen/include/asm-x86/asm-defns.h
index e164437798..505f39ad5f 100644
--- a/xen/include/asm-x86/asm-defns.h
+++ b/xen/include/asm-x86/asm-defns.h
@@ -57,22 +57,6 @@
     INDIRECT_BRANCH jmp \arg
 .endm
 
-/*
- * To guard against speculation past RET, insert a breakpoint insn
- * immediately after them.
- */
-.macro ret operand:vararg
-    retq \operand
-.endm
-.macro retq operand:vararg
-    .ifb \operand
-    .byte 0xc3
-    .else
-    .byte 0xc2
-    .word \operand
-    .endif
-.endm
-
 .macro guest_access_mask_ptr ptr:req, scratch1:req, scratch2:req
 #if defined(CONFIG_SPECULATIVE_HARDEN_GUEST_ACCESS)
     /*
--
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