Reimplement the arm64 dtrace_gethrtime(), which provides the

high-resolution nanosecond timestamp used for the DTrace 'timestamp'
built-in variable.  The new implementation uses the EL0 cycle
counter and frequency registers in ARMv8-A.  This replaces a
previous implementation that relied on an instrumentation-safe
implementation of getnanotime(), which provided only timer
resolution.

MFC after:	3 days
Reviewed by:	andrew, bsdimp (older version)
Useful comments appreciated:	jrtc27, emaste

sys/cddl/dev/dtrace/aarch64/dtrace_subr.c

@@ -153,23 +153,26 @@ dtrace_sync(void)
 }
 
 /*
- * DTrace needs a high resolution time function which can
- * be called from a probe context and guaranteed not to have
- * instrumented with probes itself.
+ * DTrace needs a high resolution time function which can be called from a
+ * probe context and guaranteed not to have instrumented with probes itself.
  *
- * Returns nanoseconds since boot.
+ * Returns nanoseconds since some arbitrary point in time (likely SoC reset?).
  */
 uint64_t
-dtrace_gethrtime()
+dtrace_gethrtime(void)
 {
-	struct timespec curtime;
-
-	dtrace_getnanouptime(&curtime);
-
-	return (curtime.tv_sec * 1000000000UL + curtime.tv_nsec);
+	uint64_t count, freq;
 
+	count = READ_SPECIALREG(cntvct_el0);
+	freq = READ_SPECIALREG(cntfrq_el0);
+	return ((1000000000UL * count) / freq);
 }
 
+/*
+ * Return a much lower resolution wallclock time based on the system clock
+ * updated by the timer.  If needed, we could add a version interpolated from
+ * the system clock as is the case with dtrace_gethrtime().
+ */
 uint64_t
 dtrace_gethrestime(void)
 {