x86/speculation: Simplify and make CALL_NOSPEC consistent

commit cfceff8526a426948b53445c02bcb98453c7330d upstream.

CALL_NOSPEC macro is used to generate Spectre-v2 mitigation friendly
indirect branches. At compile time the macro defaults to indirect branch,
and at runtime those can be patched to thunk based mitigations.

This approach is opposite of what is done for the rest of the kernel, where
the compile time default is to replace indirect calls with retpoline thunk
calls.

Make CALL_NOSPEC consistent with the rest of the kernel, default to
retpoline thunk at compile time when CONFIG_RETPOLINE is
enabled.

  [ pawan: s/CONFIG_MITIGATION_RETPOLINE/CONFIG_RETPOLINE/ ]

Signed-off-by: Pawan Gupta <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Andrew Cooper <[email protected]
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: pa1gupta

arch/x86/include/asm/nospec-branch.h

@@ -284,16 +284,11 @@ extern void (*x86_return_thunk)(void);
  * Inline asm uses the %V modifier which is only in newer GCC
  * which is ensured when CONFIG_RETPOLINE is defined.
  */
-# define CALL_NOSPEC						\
-	ALTERNATIVE_2(						\
-	ANNOTATE_RETPOLINE_SAFE					\
-	"call *%[thunk_target]\n",				\
-	"call __x86_indirect_thunk_%V[thunk_target]\n",		\
-	X86_FEATURE_RETPOLINE,					\
-	"lfence;\n"						\
-	ANNOTATE_RETPOLINE_SAFE					\
-	"call *%[thunk_target]\n",				\
-	X86_FEATURE_RETPOLINE_LFENCE)
+#ifdef CONFIG_RETPOLINE
+#define CALL_NOSPEC	"call __x86_indirect_thunk_%V[thunk_target]\n"
+#else
+#define CALL_NOSPEC	"call *%[thunk_target]\n"
+#endif
 
 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)