x86/alternatives: Add macro comments

... so that when one stares at the .s output, one can find her way
around the resulting asm magic.

With it, ALTERNATIVE looks like this now:

          # ALT: oldnstr
  661:
          ...
  662:
          # ALT: padding
  .skip ...
  663:
  .pushsection .altinstructions,"a"

  ...

  .popsection
  .pushsection .altinstr_replacement, "ax"
          # ALT: replacement 1
  6641:
  	...
  6651:
          .popsection

Merge __OLDINSTR() into OLDINSTR(), while at it.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: X86 ML <x86@kernel.org>
Link: https://lkml.kernel.org/r/20181211222326.14581-2-bp@alien8.de

arch/x86/include/asm/alternative.h

@@ -94,13 +94,12 @@ static inline int alternatives_text_reserved(void *start, void *end)
 #define alt_total_slen		alt_end_marker"b-661b"
 #define alt_rlen(num)		e_replacement(num)"f-"b_replacement(num)"f"
 
-#define __OLDINSTR(oldinstr, num)					\
+#define OLDINSTR(oldinstr, num)						\
+	"# ALT: oldnstr\n"						\
 	"661:\n\t" oldinstr "\n662:\n"					\
+	"# ALT: padding\n"						\
 	".skip -(((" alt_rlen(num) ")-(" alt_slen ")) > 0) * "		\
-		"((" alt_rlen(num) ")-(" alt_slen ")),0x90\n"
-
-#define OLDINSTR(oldinstr, num)						\
-	__OLDINSTR(oldinstr, num)					\
+		"((" alt_rlen(num) ")-(" alt_slen ")),0x90\n"		\
 	alt_end_marker ":\n"
 
 /*
@@ -116,7 +115,9 @@ static inline int alternatives_text_reserved(void *start, void *end)
  * additionally longer than the first replacement alternative.
  */
 #define OLDINSTR_2(oldinstr, num1, num2) \
+	"# ALT: oldinstr2\n"									\
 	"661:\n\t" oldinstr "\n662:\n"								\
+	"# ALT: padding2\n"									\
 	".skip -((" alt_max_short(alt_rlen(num1), alt_rlen(num2)) " - (" alt_slen ")) > 0) * "	\
 		"(" alt_max_short(alt_rlen(num1), alt_rlen(num2)) " - (" alt_slen ")), 0x90\n"	\
 	alt_end_marker ":\n"
@@ -129,8 +130,9 @@ static inline int alternatives_text_reserved(void *start, void *end)
 	" .byte " alt_rlen(num) "\n"			/* replacement len */ \
 	" .byte " alt_pad_len "\n"			/* pad len */
 
-#define ALTINSTR_REPLACEMENT(newinstr, feature, num)	/* replacement */     \
-	b_replacement(num)":\n\t" newinstr "\n" e_replacement(num) ":\n\t"
+#define ALTINSTR_REPLACEMENT(newinstr, feature, num)	/* replacement */	\
+	"# ALT: replacement " #num "\n"						\
+	b_replacement(num)":\n\t" newinstr "\n" e_replacement(num) ":\n"
 
 /* alternative assembly primitive: */
 #define ALTERNATIVE(oldinstr, newinstr, feature)			\