vdso: Introduce aux_clock_resolution_ns()

Move the constant resolution to a shared header,
so the vDSO can use it and return it without going through a syscall.

Signed-off-by: Thomas Weißschuh <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]

include/vdso/auxclock.h

···
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _VDSO_AUXCLOCK_H
+#define _VDSO_AUXCLOCK_H
+
+#include <uapi/linux/time.h>
+#include <uapi/linux/types.h>
+
+static __always_inline u64 aux_clock_resolution_ns(void)
+{
+	return 1;
+}
+
+#endif /* _VDSO_AUXCLOCK_H */

kernel/time/timekeeping.c

···
 #include <linux/audit.h>
 #include <linux/random.h>
 
+#include <vdso/auxclock.h>
+
 #include "tick-internal.h"
 #include "ntp_internal.h"
 #include "timekeeping_internal.h"
···
 	if (!clockid_aux_valid(id))
 		return -ENODEV;
 
-	tp->tv_sec = 0;
-	tp->tv_nsec = 1;
+	tp->tv_sec = aux_clock_resolution_ns() / NSEC_PER_SEC;
+	tp->tv_nsec = aux_clock_resolution_ns() % NSEC_PER_SEC;
 	return 0;
 }