s390/current: Implement current with inline assembly

Implement current with an inline assembly, which makes use of the
ALTERNATIVE macro, to read current from lowcore. Provide an alternative
instruction with a different offset in case lowcore is relocated.

This replaces sequences of two instructions with one instruction.

Before:
100076: a5 1e 00 00 llilh %r1,0
10007a: e3 40 13 40 00 04 lg %r4,832(%r1)

After:
100076: e3 10 03 40 00 04 lg %r1,832

Kernel image size change:
add/remove: 3/17 grow/shrink: 166/2204 up/down: 7122/-24594 (-17472)

Reviewed-by: Alexander Gordeev <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Vasily Gorbik <[e-mail]>

arch/s390/include/asm/current.h

···
 #define _S390_CURRENT_H
 
 #include <asm/lowcore.h>
+#include <asm/machine.h>
 
 struct task_struct;
 
-#define current ((struct task_struct *const)get_lowcore()->current_task)
+static __always_inline struct task_struct *get_current(void)
+{
+	unsigned long ptr, lc_current;
+
+	lc_current = offsetof(struct lowcore, current_task);
+	asm_inline(
+		ALTERNATIVE("	lg	%[ptr],%[offzero](%%r0)\n",
+			    "	lg	%[ptr],%[offalt](%%r0)\n",
+			    ALT_FEATURE(MFEATURE_LOWCORE))
+		: [ptr] "=d" (ptr)
+		: [offzero] "i" (lc_current),
+		  [offalt] "i" (lc_current + LOWCORE_ALT_ADDRESS));
+	return (struct task_struct *)ptr;
+}
+
+#define current get_current()
 
 #endif /* !(_S390_CURRENT_H) */