x86/alternatives: Optimize optimize_nops()

Currently, optimize_nops() scans to see if the alternative starts with
NOPs. However, the emit pattern is:

  141:	\oldinstr
  142:	.skip (len-(142b-141b)), 0x90

That is, when 'oldinstr' is short, the tail is padded with NOPs. This case
never gets optimized.

Rewrite optimize_nops() to replace any trailing string of NOPs inside
the alternative to larger NOPs. Also run it irrespective of patching,
replacing NOPs in both the original and replaced code.

A direct consequence is that 'padlen' becomes superfluous, so remove it.

 [ bp:
   - Adjust commit message
   - remove a stale comment about needing to pad
   - add a comment in optimize_nops()
   - exit early if the NOP verif. loop catches a mismatch - function
     should not not add NOPs in that case
   - fix the "optimized NOPs" offsets output ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210326151259.442992235@infradead.org

1 files changed, 5 insertions, 12 deletions

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 17b36090d448..a3c2315aca12 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -65,7 +65,6 @@ struct alt_instr {
 	u16 cpuid;		/* cpuid bit set for replacement */
 	u8  instrlen;		/* length of original instruction */
 	u8  replacementlen;	/* length of new instruction */
-	u8  padlen;		/* length of build-time padding */
 } __packed;
 
 /*
@@ -104,7 +103,6 @@ static inline int alternatives_text_reserved(void *start, void *end)
 
 #define alt_end_marker		"663"
 #define alt_slen		"662b-661b"
-#define alt_pad_len		alt_end_marker"b-662b"
 #define alt_total_slen		alt_end_marker"b-661b"
 #define alt_rlen(num)		e_replacement(num)"f-"b_replacement(num)"f"
 
@@ -151,8 +149,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
 	" .long " b_replacement(num)"f - .\n"		/* new instruction */ \
 	" .word " __stringify(feature) "\n"		/* feature bit     */ \
 	" .byte " alt_total_slen "\n"			/* source len      */ \
-	" .byte " alt_rlen(num) "\n"			/* replacement len */ \
-	" .byte " alt_pad_len "\n"			/* pad len */
+	" .byte " alt_rlen(num) "\n"			/* replacement len */
 
 #define ALTINSTR_REPLACEMENT(newinstr, num)		/* replacement */	\
 	"# ALT: replacement " #num "\n"						\
@@ -224,9 +221,6 @@ static inline int alternatives_text_reserved(void *start, void *end)
  * Peculiarities:
  * No memory clobber here.
  * Argument numbers start with 1.
- * Best is to use constraints that are fixed size (like (%1) ... "r")
- * If you use variable sized constraints like "m" or "g" in the
- * replacement make sure to pad to the worst case length.
  * Leaving an unused argument 0 to keep API compatibility.
  */
 #define alternative_input(oldinstr, newinstr, feature, input...)	\
@@ -315,13 +309,12 @@ static inline int alternatives_text_reserved(void *start, void *end)
  * enough information for the alternatives patching code to patch an
  * instruction. See apply_alternatives().
  */
-.macro altinstruction_entry orig alt feature orig_len alt_len pad_len
+.macro altinstruction_entry orig alt feature orig_len alt_len
 	.long \orig - .
 	.long \alt - .
 	.word \feature
 	.byte \orig_len
 	.byte \alt_len
-	.byte \pad_len
 .endm
 
 /*
@@ -338,7 +331,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
 142:
 
 	.pushsection .altinstructions,"a"
-	altinstruction_entry 140b,143f,\feature,142b-140b,144f-143f,142b-141b
+	altinstruction_entry 140b,143f,\feature,142b-140b,144f-143f
 	.popsection
 
 	.pushsection .altinstr_replacement,"ax"
@@ -375,8 +368,8 @@ static inline int alternatives_text_reserved(void *start, void *end)
 142:
 
 	.pushsection .altinstructions,"a"
-	altinstruction_entry 140b,143f,\feature1,142b-140b,144f-143f,142b-141b
-	altinstruction_entry 140b,144f,\feature2,142b-140b,145f-144f,142b-141b
+	altinstruction_entry 140b,143f,\feature1,142b-140b,144f-143f
+	altinstruction_entry 140b,144f,\feature2,142b-140b,145f-144f
 	.popsection
 
 	.pushsection .altinstr_replacement,"ax"