x86/speculation: Change FILL_RETURN_BUFFER to work with objtool

Change FILL_RETURN_BUFFER so that objtool groks it and can generate
correct ORC unwind information.

 - Since ORC is alternative invariant; that is, all alternatives
   should have the same ORC entries, the __FILL_RETURN_BUFFER body
   can not be part of an alternative.

   Therefore, move it out of the alternative and keep the alternative
   as a sort of jump_label around it.

 - Use the ANNOTATE_INTRA_FUNCTION_CALL annotation to white-list
   these 'funny' call instructions to nowhere.

 - Use UNWIND_HINT_EMPTY to 'fill' the speculation traps, otherwise
   objtool will consider them unreachable.

 - Move the RSP adjustment into the loop, such that the loop has a
   deterministic stack layout.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20200428191700.032079304@infradead.org

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

@@ -4,11 +4,13 @@
 #define _ASM_X86_NOSPEC_BRANCH_H_
 
 #include <linux/static_key.h>
+#include <linux/frame.h>
 
 #include <asm/alternative.h>
 #include <asm/alternative-asm.h>
 #include <asm/cpufeatures.h>
 #include <asm/msr-index.h>
+#include <asm/unwind_hints.h>
 
 /*
  * This should be used immediately before a retpoline alternative. It tells
@@ -46,21 +48,25 @@
 #define __FILL_RETURN_BUFFER(reg, nr, sp)	\
 	mov	$(nr/2), reg;			\
 771:						\
+	ANNOTATE_INTRA_FUNCTION_CALL;		\
 	call	772f;				\
 773:	/* speculation trap */			\
+	UNWIND_HINT_EMPTY;			\
 	pause;					\
 	lfence;					\
 	jmp	773b;				\
 772:						\
+	ANNOTATE_INTRA_FUNCTION_CALL;		\
 	call	774f;				\
 775:	/* speculation trap */			\
+	UNWIND_HINT_EMPTY;			\
 	pause;					\
 	lfence;					\
 	jmp	775b;				\
 774:						\
+	add	$(BITS_PER_LONG/8) * 2, sp;	\
 	dec	reg;				\
-	jnz	771b;				\
-	add	$(BITS_PER_LONG/8) * nr, sp;
+	jnz	771b;
 
 #ifdef __ASSEMBLY__
 
@@ -137,10 +143,8 @@
   */
 .macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
 #ifdef CONFIG_RETPOLINE
-	ANNOTATE_NOSPEC_ALTERNATIVE
-	ALTERNATIVE "jmp .Lskip_rsb_\@",				\
-		__stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP))	\
-		\ftr
+	ALTERNATIVE "jmp .Lskip_rsb_\@", "", \ftr
+	__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP)
 .Lskip_rsb_\@:
 #endif
 .endm