x86/percpu: Clean up percpu_xchg_op()

The core percpu macros already have a switch on the data size, so the switch
in the x86 code is redundant and produces more dead code.

Also use appropriate types for the width of the instructions.  This avoids
errors when compiling with Clang.
Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Dennis Zhou <dennis@kernel.org>
Link: https://lkml.kernel.org/r/20200720204925.3654302-8-ndesaulniers@google.com

arch/x86/include/asm/percpu.h

@@ -215,46 +215,21 @@ do {									\
  * expensive due to the implied lock prefix.  The processor cannot prefetch
  * cachelines if xchg is used.
  */
-#define percpu_xchg_op(qual, var, nval)					\
+#define percpu_xchg_op(size, qual, _var, _nval)				\
 ({									\
-	typeof(var) pxo_ret__;						\
-	typeof(var) pxo_new__ = (nval);					\
-	switch (sizeof(var)) {						\
-	case 1:								\
-		asm qual ("\n\tmov "__percpu_arg(1)",%%al"		\
-		    "\n1:\tcmpxchgb %2, "__percpu_arg(1)		\
-		    "\n\tjnz 1b"					\
-			    : "=&a" (pxo_ret__), "+m" (var)		\
-			    : "q" (pxo_new__)				\
-			    : "memory");				\
-		break;							\
-	case 2:								\
-		asm qual ("\n\tmov "__percpu_arg(1)",%%ax"		\
-		    "\n1:\tcmpxchgw %2, "__percpu_arg(1)		\
-		    "\n\tjnz 1b"					\
-			    : "=&a" (pxo_ret__), "+m" (var)		\
-			    : "r" (pxo_new__)				\
-			    : "memory");				\
-		break;							\
-	case 4:								\
-		asm qual ("\n\tmov "__percpu_arg(1)",%%eax"		\
-		    "\n1:\tcmpxchgl %2, "__percpu_arg(1)		\
-		    "\n\tjnz 1b"					\
-			    : "=&a" (pxo_ret__), "+m" (var)		\
-			    : "r" (pxo_new__)				\
-			    : "memory");				\
-		break;							\
-	case 8:								\
-		asm qual ("\n\tmov "__percpu_arg(1)",%%rax"		\
-		    "\n1:\tcmpxchgq %2, "__percpu_arg(1)		\
-		    "\n\tjnz 1b"					\
-			    : "=&a" (pxo_ret__), "+m" (var)		\
-			    : "r" (pxo_new__)				\
-			    : "memory");				\
-		break;							\
-	default: __bad_percpu_size();					\
-	}								\
-	pxo_ret__;							\
+	__pcpu_type_##size pxo_old__;					\
+	__pcpu_type_##size pxo_new__ = __pcpu_cast_##size(_nval);	\
+	asm qual (__pcpu_op2_##size("mov", __percpu_arg([var]),		\
+				    "%[oval]")				\
+		  "\n1:\t"						\
+		  __pcpu_op2_##size("cmpxchg", "%[nval]",		\
+				    __percpu_arg([var]))		\
+		  "\n\tjnz 1b"						\
+		  : [oval] "=&a" (pxo_old__),				\
+		    [var] "+m" (_var)					\
+		  : [nval] __pcpu_reg_##size(, pxo_new__)		\
+		  : "memory");						\
+	(typeof(_var))(unsigned long) pxo_old__;			\
 })
 
 /*
@@ -354,9 +329,9 @@ do {									\
 #define this_cpu_or_1(pcp, val)		percpu_to_op(1, volatile, "or", (pcp), val)
 #define this_cpu_or_2(pcp, val)		percpu_to_op(2, volatile, "or", (pcp), val)
 #define this_cpu_or_4(pcp, val)		percpu_to_op(4, volatile, "or", (pcp), val)
-#define this_cpu_xchg_1(pcp, nval)	percpu_xchg_op(volatile, pcp, nval)
-#define this_cpu_xchg_2(pcp, nval)	percpu_xchg_op(volatile, pcp, nval)
-#define this_cpu_xchg_4(pcp, nval)	percpu_xchg_op(volatile, pcp, nval)
+#define this_cpu_xchg_1(pcp, nval)	percpu_xchg_op(1, volatile, pcp, nval)
+#define this_cpu_xchg_2(pcp, nval)	percpu_xchg_op(2, volatile, pcp, nval)
+#define this_cpu_xchg_4(pcp, nval)	percpu_xchg_op(4, volatile, pcp, nval)
 
 #define raw_cpu_add_return_1(pcp, val)		percpu_add_return_op(1, , pcp, val)
 #define raw_cpu_add_return_2(pcp, val)		percpu_add_return_op(2, , pcp, val)
@@ -409,7 +384,7 @@ do {									\
 #define this_cpu_and_8(pcp, val)		percpu_to_op(8, volatile, "and", (pcp), val)
 #define this_cpu_or_8(pcp, val)			percpu_to_op(8, volatile, "or", (pcp), val)
 #define this_cpu_add_return_8(pcp, val)		percpu_add_return_op(8, volatile, pcp, val)
-#define this_cpu_xchg_8(pcp, nval)		percpu_xchg_op(volatile, pcp, nval)
+#define this_cpu_xchg_8(pcp, nval)		percpu_xchg_op(8, volatile, pcp, nval)
 #define this_cpu_cmpxchg_8(pcp, oval, nval)	percpu_cmpxchg_op(volatile, pcp, oval, nval)
 
 /*