[PATCH v3 6/7] x86: make guarding against straight-line speculation optional

[Jan Beulich <jbeulich@xxxxxxxx>]

Put insertion of INT3 behind CONFIG_SPECULATIVE_HARDEN_BRANCH
conditionals.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v3: New.

--- a/xen/arch/x86/indirect-thunk.S
+++ b/xen/arch/x86/indirect-thunk.S
@@ -11,8 +11,10 @@
 
 #include <asm/asm_defns.h>
 
+#ifdef CONFIG_SPECULATIVE_HARDEN_BRANCH
 /* Don't transform the "ret" further down. */
 .purgem ret
+#endif
 
 .macro IND_THUNK_RETPOLINE reg:req
         call 2f
@@ -45,7 +47,9 @@ ENTRY(__x86_indirect_thunk_\reg)
         __stringify(IND_THUNK_LFENCE \reg), X86_FEATURE_IND_THUNK_LFENCE, \
         __stringify(IND_THUNK_JMP \reg),    X86_FEATURE_IND_THUNK_JMP
 
+#ifdef CONFIG_SPECULATIVE_HARDEN_BRANCH
         int3 /* Halt straight-line speculation */
+#endif
 
         .size __x86_indirect_thunk_\reg, . - __x86_indirect_thunk_\reg
         .type __x86_indirect_thunk_\reg, @function
--- a/xen/include/asm-x86/asm-defns.h
+++ b/xen/include/asm-x86/asm-defns.h
@@ -45,6 +45,8 @@
     INDIRECT_BRANCH jmp \arg
 .endm
 
+#ifdef CONFIG_SPECULATIVE_HARDEN_BRANCH
+
 /*
  * To guard against speculation past RET, insert a breakpoint insn
  * immediately after them.
@@ -60,3 +62,5 @@
     .word \operand
     .endif
 .endm
+
+#endif