Redo Kernel NTP PLL support, kernel side.

This code is mostly taken from the 1.1 port (which was in turn taken from
Dave Mills's kern.tar.Z example).  A few significant differences:

1) ntp_gettime() is now a MIB variable rather than a system call.  A few
fiddles are done in libc to make it behave the same.

2) mono_time does not participate in the PLL adjustments.

3) A new interface has been defined (in <machine/clock.h>) for doing
possibly machine-dependent things around the time of the clock update.
This is used in Pentium kernels to disable interrupts, set `time', and
reset the CPU cycle counter as quickly as possible to avoid jitter in
microtime().  Measurements show an apparent resolution of a bit more than
8.14usec, which is reasonable given system-call overhead.

1 files changed, 49 insertions, 0 deletions

diff --git a/sys/i386/include/clock.h b/sys/i386/include/clock.h
new file mode 100644
index 0000000..b7420e4
--- /dev/null
+++ b/sys/i386/include/clock.h
@@ -0,0 +1,49 @@
+/*
+ * Kernel interface to machine-dependent clock driver.
+ * Garrett Wollman, September 1994.
+ * This file is in the public domain.
+ */
+
+#ifndef _MACHINE_CLOCK_H_
+#define _MACHINE_CLOCK_H_ 1
+
+extern int pentium_mhz;
+
+#ifdef I586_CPU
+	/*
+	 * This resets the CPU cycle counter to zero, to make our
+	 * job easier in microtime().  Some fancy ifdefs could speed
+	 * this up for Pentium-only kernels.
+	 * We want this to be done as close as possible to the actual
+	 * timer incrementing in hardclock(), because there is a window
+	 * between the two where the value is no longer valid.  Experimentation
+	 * may reveal a good precompensation to apply in microtime().
+	 */
+#define CPU_CLOCKUPDATE(otime, ntime) \
+	do { \
+	if(pentium_mhz) { \
+		__asm __volatile("cli\n" \
+				 "movl (%2),%%eax\n" \
+				 "movl %%eax,(%1)\n" \
+				 "movl 4(%2),%%eax\n" \
+				 "movl %%eax,4(%1)\n" \
+				 "movl $0x10,%%ecx\n" \
+				 "xorl %%eax,%%eax\n" \
+				 "movl %%eax,%%edx\n" \
+				 ".byte 0x0f, 0x30\n" \
+				 "sti\n" \
+				 "#%0%1%2" \
+				 : "=m"(*otime)	/* no outputs */ \
+				 : "c"(otime), "b"(ntime) /* fake input */ \
+				 : "ax", "cx", "dx"); \
+	} else { \
+		*(otime) = *(ntime); \
+	} \
+	} while(0)
+
+#else
+#define CPU_CLOCKUPDATE(otime, ntime) \
+		(*(otime) = *(ntime))
+#endif
+
+#endif /* _MACHINE_CLOCK_H_ */