x86: add comments for NOPs

Add comments describing the various NOP sequences.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 45 insertions, 17 deletions

diff --git a/include/asm-x86/nops.h b/include/asm-x86/nops.h
index c52b334b8166..e3b2bce0aff8 100644
--- a/include/asm-x86/nops.h
+++ b/include/asm-x86/nops.h
@@ -3,17 +3,29 @@
 
 /* Define nops for use with alternative() */
 
-/* generic versions from gas */
-#define GENERIC_NOP1    ".byte 0x90\n"
-#define GENERIC_NOP2            ".byte 0x89,0xf6\n"
-#define GENERIC_NOP3        ".byte 0x8d,0x76,0x00\n"
-#define GENERIC_NOP4        ".byte 0x8d,0x74,0x26,0x00\n"
-#define GENERIC_NOP5        GENERIC_NOP1 GENERIC_NOP4
-#define GENERIC_NOP6    ".byte 0x8d,0xb6,0x00,0x00,0x00,0x00\n"
-#define GENERIC_NOP7    ".byte 0x8d,0xb4,0x26,0x00,0x00,0x00,0x00\n"
-#define GENERIC_NOP8    GENERIC_NOP1 GENERIC_NOP7
+/* generic versions from gas
+   1: nop
+   2: movl %esi,%esi
+   3: leal 0x00(%esi),%esi
+   4: leal 0x00(,%esi,1),%esi
+   6: leal 0x00000000(%esi),%esi
+   7: leal 0x00000000(,%esi,1),%esi
+*/
+#define GENERIC_NOP1 ".byte 0x90\n"
+#define GENERIC_NOP2 ".byte 0x89,0xf6\n"
+#define GENERIC_NOP3 ".byte 0x8d,0x76,0x00\n"
+#define GENERIC_NOP4 ".byte 0x8d,0x74,0x26,0x00\n"
+#define GENERIC_NOP5 GENERIC_NOP1 GENERIC_NOP4
+#define GENERIC_NOP6 ".byte 0x8d,0xb6,0x00,0x00,0x00,0x00\n"
+#define GENERIC_NOP7 ".byte 0x8d,0xb4,0x26,0x00,0x00,0x00,0x00\n"
+#define GENERIC_NOP8 GENERIC_NOP1 GENERIC_NOP7
 
-/* Opteron 64bit nops */
+/* Opteron 64bit nops
+   1: nop
+   2: osp nop
+   3: osp osp nop
+   4: osp osp osp nop
+*/
 #define K8_NOP1 GENERIC_NOP1
 #define K8_NOP2 ".byte 0x66,0x90\n"
 #define K8_NOP3 ".byte 0x66,0x66,0x90\n"
@@ -23,19 +35,35 @@
 #define K8_NOP7 K8_NOP4 K8_NOP3
 #define K8_NOP8 K8_NOP4 K8_NOP4
 
-/* K7 nops */
-/* uses eax dependencies (arbitary choice) */
-#define K7_NOP1  GENERIC_NOP1
+/* K7 nops
+   uses eax dependencies (arbitary choice)
+   1: nop
+   2: movl %eax,%eax
+   3: leal (,%eax,1),%eax
+   4: leal 0x00(,%eax,1),%eax
+   6: leal 0x00000000(%eax),%eax
+   7: leal 0x00000000(,%eax,1),%eax
+*/
+#define K7_NOP1 GENERIC_NOP1
 #define K7_NOP2 ".byte 0x8b,0xc0\n"
 #define K7_NOP3 ".byte 0x8d,0x04,0x20\n"
 #define K7_NOP4 ".byte 0x8d,0x44,0x20,0x00\n"
 #define K7_NOP5 K7_NOP4 ASM_NOP1
 #define K7_NOP6 ".byte 0x8d,0x80,0,0,0,0\n"
-#define K7_NOP7        ".byte 0x8D,0x04,0x05,0,0,0,0\n"
-#define K7_NOP8        K7_NOP7 ASM_NOP1
+#define K7_NOP7 ".byte 0x8D,0x04,0x05,0,0,0,0\n"
+#define K7_NOP8 K7_NOP7 ASM_NOP1
 
-/* P6 nops */
-/* uses eax dependencies (Intel-recommended choice) */
+/* P6 nops
+   uses eax dependencies (Intel-recommended choice)
+   1: nop
+   2: osp nop
+   3: nopl (%eax)
+   4: nopl 0x00(%eax)
+   5: nopl 0x00(%eax,%eax,1)
+   6: osp nopl 0x00(%eax,%eax,1)
+   7: nopl 0x00000000(%eax)
+   8: nopl 0x00000000(%eax,%eax,1)
+*/
 #define P6_NOP1 GENERIC_NOP1
 #define P6_NOP2 ".byte 0x66,0x90\n"
 #define P6_NOP3 ".byte 0x0f,0x1f,0x00\n"