[PATCH] recordmcount: arm: Implement make_nop

On 01/30, Rabin Vincent wrote:

On Fri, Jan 29, 2016 at 05:18:06PM -0800, Stephen Boyd wrote:

quoted

diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
index e167592793a7..0b16d14c54fb 100644
--- a/scripts/recordmcount.c
+++ b/scripts/recordmcount.c

@@ -206,6 +206,52 @@ static int make_nop_x86(void *map, size_t const offset)
 	return 0;
 }
 
+/*
+ * Indicates if ARM is using __gnu_mcount_nc or mcount style and if
+ * we should replace it with a pop or a nop respectively.
+ */

For __gnu_mcount_nc, wouldn't it be better to replace both the push {lr}
and the bl with nop instructions, instead of keeping a (useless) push +
pop sequence?

Agreed. I was trying to do a 1-to-1 copy of the ftrace code on
ARM.  I was wondering the same thing in that code path while
doing this though. Can't we replace both instructions instead of
one instruction when we're patching in nops at runtime?

quoted

+static int uses_altmcount;
+
+static unsigned char ideal_nop4_arm_arm[4] = { 0x00, 0x40, 0xbd, 0xe8 };
+static unsigned char ideal_nop4_arm_thumb[4] = { 0x5d, 0xf8, 0x04, 0xeb };
+static unsigned char ideal_nop4_arm_arm_be[4] = { 0xe8, 0xbd, 0x40, 0x00 };
+static unsigned char ideal_nop4_arm_thumb_be[4] = { 0xf8, 0x5d, 0xeb, 0x04 };
+static unsigned char ideal_nop4_arm_old[4] = { 0x00, 0x00, 0xa0, 0xe1 };
+static unsigned char ideal_nop4_arm_old_be[4] = { 0xe1, 0xa0, 0x00, 0x00 };
+
+static unsigned char bl_gnu_mcount_nc_arm[4] = { 0xfe, 0xff, 0xff, 0xeb };
+static unsigned char bl_gnu_mcount_nc_thumb[4] = { 0xff, 0xf7, 0xfe, 0xff };
+static unsigned char bl_gnu_mcount_nc_arm_be[4] = { 0xeb, 0xff, 0xff, 0xfe };
+static unsigned char bl_gnu_mcount_nc_thumb_be[4] = { 0xf7, 0xff, 0xff, 0xfe };

Comments showing what assembly instructions all these correspond to
would be helpful.

Sure.

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