[PATCH] x86/percpu: Use correct asm operand modifier in percpu_stable_op

[Uros Bizjak]

"P" asm operand modifier is a x86 target-specific modifier. When used
for a constant, it drops all syntax-specific prefixes and issues the
bare constant. This modifier is not correct for address handling,
in this case a generic "a" operand modifier should be used.

"a" asm operand modifier substitutes a nemory reference, with the
actual operand treated as address.  For x86_64, when a symbol is
provided, the "a" modifier emits "sym(%rip)" instead of "sym",
enabling shorter %rip-relative addressing.

Clang allows only "i" and "r" opreand constraints with "a" modifier,
so the patch normalizes the modifier/constraint pair to "a"/"i"
which is consistent between both compilers.

The patch reduces code size of a test build for 4072 bytes:

25519196        4388300  808452 30715948        1d4b02c vmlinux-new.o
25523268        4388300  808452 30720020        1d4c014 vmlinux-old.o

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
 arch/x86/include/asm/percpu.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 54746903b8c3..ac3220aeb779 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -194,9 +194,9 @@ do {									\
 #define percpu_stable_op(size, op, _var)				\
 ({									\
 	__pcpu_type_##size pfo_val__;					\
-	asm(__pcpu_op2_##size(op, __force_percpu_arg(P[var]), "%[val]")	\
+	asm(__pcpu_op2_##size(op, __force_percpu_arg(a[var]), "%[val]")	\
 	    : [val] __pcpu_reg_##size("=", pfo_val__)			\
-	    : [var] "p" (&(_var)));					\
+	    : [var] "i" (&(_var)));					\
 	(typeof(_var))(unsigned long) pfo_val__;			\
 })
 
-- 
2.41.0