svn commit: r350178 - head/sys/amd64/vmm/io

[John Baldwin]

Author: jhb
Date: Sat Jul 20 15:59:49 2019
New Revision: 350178
URL: https://svnweb.freebsd.org/changeset/base/350178

Log:
  Improve the precision of bhyve's vPIT.
  
  Use 'struct bintime' instead of 'sbintime_t' to manage times in vPIT
  to postpone rounding to final results rather than intermediate
  results.  In tests performed by Joyent, this reduced the error measured
  by Linux guests by 59 ppm.
  
  Smart OS bug:	https://smartos.org/bugview/OS-6923
  Submitted by:	Patrick Mooney
  Reviewed by:	rgrimes
  Obtained from:	SmartOS / Joyent
  MFC after:	2 weeks
  Differential Revision:	https://reviews.freebsd.org/D20335

Modified:
  head/sys/amd64/vmm/io/vatpit.c

Modified: head/sys/amd64/vmm/io/vatpit.c
==============================================================================
--- head/sys/amd64/vmm/io/vatpit.c	Sat Jul 20 15:26:21 2019	(r350177)
+++ head/sys/amd64/vmm/io/vatpit.c	Sat Jul 20 15:59:49 2019	(r350178)
@@ -2,6 +2,7 @@
  * Copyright (c) 2014 Tycho Nightingale <tycho.nightingale at pluribusnetworks.com>
  * Copyright (c) 2011 NetApp, Inc.
  * All rights reserved.
+ * Copyright (c) 2018 Joyent, Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -78,7 +79,7 @@ struct vatpit_callout_arg {
 struct channel {
 	int		mode;
 	uint16_t	initial;	/* initial counter value */
-	sbintime_t	now_sbt;	/* uptime when counter was loaded */
+	struct bintime	now_bt;		/* uptime when counter was loaded */
 	uint8_t		cr[2];
 	uint8_t		ol[2];
 	bool		slatched;	/* status latched */
@@ -87,7 +88,7 @@ struct channel {
 	int		olbyte;
 	int		frbyte;
 	struct callout	callout;
-	sbintime_t	callout_sbt;	/* target time */
+	struct bintime	callout_bt;	/* target time */
 	struct vatpit_callout_arg callout_arg;
 };
 
@@ -95,26 +96,41 @@ struct vatpit {
 	struct vm	*vm;
 	struct mtx	mtx;
 
-	sbintime_t	freq_sbt;
+	struct bintime	freq_bt;
 
 	struct channel	channel[3];
 };
 
 static void pit_timer_start_cntr0(struct vatpit *vatpit);
 
+static uint64_t
+vatpit_delta_ticks(struct vatpit *vatpit, struct channel *c)
+{
+	struct bintime delta;
+	uint64_t result;
+
+	binuptime(&delta);
+	bintime_sub(&delta, &c->now_bt);
+
+	result = delta.sec * PIT_8254_FREQ;
+	result += delta.frac / vatpit->freq_bt.frac;
+
+	return (result);
+}
+
 static int
 vatpit_get_out(struct vatpit *vatpit, int channel)
 {
 	struct channel *c;
-	sbintime_t delta_ticks;
+	uint64_t delta_ticks;
 	int out;
 
 	c = &vatpit->channel[channel];
 
 	switch (c->mode) {
 	case TIMER_INTTC:
-		delta_ticks = (sbinuptime() - c->now_sbt) / vatpit->freq_sbt;
-		out = ((c->initial - delta_ticks) <= 0);
+		delta_ticks = vatpit_delta_ticks(vatpit, c);
+		out = (delta_ticks >= c->initial);
 		break;
 	default:
 		out = 0;
@@ -164,24 +180,28 @@ static void
 pit_timer_start_cntr0(struct vatpit *vatpit)
 {
 	struct channel *c;
-	sbintime_t now, delta, precision;
+	struct bintime now, delta;
+	sbintime_t precision;
 
 	c = &vatpit->channel[0];
 	if (c->initial != 0) {
-		delta = c->initial * vatpit->freq_sbt;
-		precision = delta >> tc_precexp;
-		c->callout_sbt = c->callout_sbt + delta;
+		delta.sec = 0;
+		delta.frac = vatpit->freq_bt.frac * c->initial;
+		bintime_add(&c->callout_bt, &delta);
+		precision = bttosbt(delta) >> tc_precexp;
 
 		/*
-		 * Reset 'callout_sbt' if the time that the callout
+		 * Reset 'callout_bt' if the time that the callout
 		 * was supposed to fire is more than 'c->initial'
 		 * ticks in the past.
 		 */
-		now = sbinuptime();
-		if (c->callout_sbt < now)
-			c->callout_sbt = now + delta;
+		binuptime(&now);
+		if (bintime_cmp(&c->callout_bt, &now, <)) {
+			c->callout_bt = now;
+			bintime_add(&c->callout_bt, &delta);
+		}
 
-		callout_reset_sbt(&c->callout, c->callout_sbt,
+		callout_reset_sbt(&c->callout, bttosbt(c->callout_bt),
 		    precision, vatpit_callout_handler, &c->callout_arg,
 		    C_ABSOLUTE);
 	}
@@ -191,7 +211,7 @@ static uint16_t
 pit_update_counter(struct vatpit *vatpit, struct channel *c, bool latch)
 {
 	uint16_t lval;
-	sbintime_t delta_ticks;
+	uint64_t delta_ticks;
 
 	/* cannot latch a new value until the old one has been consumed */
 	if (latch && c->olbyte != 0)
@@ -207,12 +227,11 @@ pit_update_counter(struct vatpit *vatpit, struct chann
 		 * here.
 		 */
 		c->initial = TIMER_DIV(PIT_8254_FREQ, 100);
-		c->now_sbt = sbinuptime();
+		binuptime(&c->now_bt);
 		c->status &= ~TIMER_STS_NULLCNT;
 	}
 
-	delta_ticks = (sbinuptime() - c->now_sbt) / vatpit->freq_sbt;
-
+	delta_ticks = vatpit_delta_ticks(vatpit, c);
 	lval = c->initial - delta_ticks % c->initial;
 
 	if (latch) {
@@ -383,10 +402,10 @@ vatpit_handler(struct vm *vm, int vcpuid, bool in, int
 			c->frbyte = 0;
 			c->crbyte = 0;
 			c->initial = c->cr[0] | (uint16_t)c->cr[1] << 8;
-			c->now_sbt = sbinuptime();
+			binuptime(&c->now_bt);
 			/* Start an interval timer for channel 0 */
 			if (port == TIMER_CNTR0) {
-				c->callout_sbt = c->now_sbt;
+				c->callout_bt = c->now_bt;
 				pit_timer_start_cntr0(vatpit);
 			}
 			if (c->initial == 0)
@@ -423,7 +442,6 @@ struct vatpit *
 vatpit_init(struct vm *vm)
 {
 	struct vatpit *vatpit;
-	struct bintime bt;
 	struct vatpit_callout_arg *arg;
 	int i;
 
@@ -432,8 +450,7 @@ vatpit_init(struct vm *vm)
 
 	mtx_init(&vatpit->mtx, "vatpit lock", NULL, MTX_SPIN);
 
-	FREQ2BT(PIT_8254_FREQ, &bt);
-	vatpit->freq_sbt = bttosbt(bt);
+	FREQ2BT(PIT_8254_FREQ, &vatpit->freq_bt);
 
 	for (i = 0; i < 3; i++) {
 		callout_init(&vatpit->channel[i].callout, 1);