4 files changed, 57 insertions, 24 deletions

diff --git a/drivers/lguest/hypercalls.c b/drivers/lguest/hypercalls.c
index 7a5299f9679..db6caace3b9 100644
--- a/drivers/lguest/hypercalls.c
+++ b/drivers/lguest/hypercalls.c
@@ -64,14 +64,6 @@ static void do_hcall(struct lguest *lg, struct lguest_regs *regs)
 		else
 			guest_pagetable_flush_user(lg);
 		break;
-	case LHCALL_GET_WALLCLOCK: {
-		/* The Guest wants to know the real time in seconds since 1970,
-		 * in good Unix tradition. */
-		struct timespec ts;
-		ktime_get_real_ts(&ts);
-		regs->eax = ts.tv_sec;
-		break;
-	}
 	case LHCALL_BIND_DMA:
 		/* BIND_DMA really wants four arguments, but it's the only call
 		 * which does.  So the Guest packs the number of buffers and
@@ -235,6 +227,9 @@ static void initialize(struct lguest *lg)
 	    || put_user(lg->guestid, &lg->lguest_data->guestid))
 		kill_guest(lg, "bad guest page %p", lg->lguest_data);
 
+	/* We write the current time into the Guest's data page once now. */
+	write_timestamp(lg);
+
 	/* This is the one case where the above accesses might have been the
 	 * first write to a Guest page.  This may have caused a copy-on-write
 	 * fault, but the Guest might be referring to the old (read-only)
@@ -293,3 +288,13 @@ void do_hypercalls(struct lguest *lg)
 		clear_hcall(lg);
 	}
 }
+
+/* This routine supplies the Guest with time: it's used for wallclock time at
+ * initial boot and as a rough time source if the TSC isn't available. */
+void write_timestamp(struct lguest *lg)
+{
+	struct timespec now;
+	ktime_get_real_ts(&now);
+	if (put_user(now, &lg->lguest_data->time))
+		kill_guest(lg, "Writing timestamp");
+}
diff --git a/drivers/lguest/interrupts_and_traps.c b/drivers/lguest/interrupts_and_traps.c
index bd0091bf79e..49787e964a0 100644
--- a/drivers/lguest/interrupts_and_traps.c
+++ b/drivers/lguest/interrupts_and_traps.c
@@ -175,6 +175,13 @@ void maybe_do_interrupt(struct lguest *lg)
 		 * the stack as well: virtual interrupts never do. */
 		set_guest_interrupt(lg, idt->a, idt->b, 0);
 	}
+
+	/* Every time we deliver an interrupt, we update the timestamp in the
+	 * Guest's lguest_data struct.  It would be better for the Guest if we
+	 * did this more often, but it can actually be quite slow: doing it
+	 * here is a compromise which means at least it gets updated every
+	 * timer interrupt. */
+	write_timestamp(lg);
 }
 
 /*H:220 Now we've got the routines to deliver interrupts, delivering traps
diff --git a/drivers/lguest/lg.h b/drivers/lguest/lg.h
index 269116eee85..64f0abed317 100644
--- a/drivers/lguest/lg.h
+++ b/drivers/lguest/lg.h
@@ -256,6 +256,7 @@ unsigned long get_dma_buffer(struct lguest *lg, unsigned long key,
 
 /* hypercalls.c: */
 void do_hypercalls(struct lguest *lg);
+void write_timestamp(struct lguest *lg);
 
 /*L:035
  * Let's step aside for the moment, to study one important routine that's used
diff --git a/drivers/lguest/lguest.c b/drivers/lguest/lguest.c
index 3386b0e7690..1bc1546c7fd 100644
--- a/drivers/lguest/lguest.c
+++ b/drivers/lguest/lguest.c
@@ -643,21 +643,42 @@ static void __init lguest_init_IRQ(void)
  * Time.
  *
  * It would be far better for everyone if the Guest had its own clock, but
- * until then it must ask the Host for the time.
+ * until then the Host gives us the time on every interrupt.
  */
 static unsigned long lguest_get_wallclock(void)
 {
-	return hcall(LHCALL_GET_WALLCLOCK, 0, 0, 0);
+	return lguest_data.time.tv_sec;
 }
 
-/* If the Host tells us we can trust the TSC, we use that, otherwise we simply
- * use the imprecise but reliable "jiffies" counter. */
 static cycle_t lguest_clock_read(void)
 {
+	unsigned long sec, nsec;
+
+	/* If the Host tells the TSC speed, we can trust that. */
 	if (lguest_data.tsc_khz)
 		return native_read_tsc();
-	else
-		return jiffies;
+
+	/* If we can't use the TSC, we read the time value written by the Host.
+	 * Since it's in two parts (seconds and nanoseconds), we risk reading
+	 * it just as it's changing from 99 & 0.999999999 to 100 and 0, and
+	 * getting 99 and 0.  As Linux tends to come apart under the stress of
+	 * time travel, we must be careful: */
+	do {
+		/* First we read the seconds part. */
+		sec = lguest_data.time.tv_sec;
+		/* This read memory barrier tells the compiler and the CPU that
+		 * this can't be reordered: we have to complete the above
+		 * before going on. */
+		rmb();
+		/* Now we read the nanoseconds part. */
+		nsec = lguest_data.time.tv_nsec;
+		/* Make sure we've done that. */
+		rmb();
+		/* Now if the seconds part has changed, try again. */
+	} while (unlikely(lguest_data.time.tv_sec != sec));
+
+	/* Our non-TSC clock is in real nanoseconds. */
+	return sec*1000000000ULL + nsec;
 }
 
 /* This is what we tell the kernel is our clocksource.  */
@@ -665,8 +686,11 @@ static struct clocksource lguest_clock = {
 	.name		= "lguest",
 	.rating		= 400,
 	.read		= lguest_clock_read,
+	.mask		= CLOCKSOURCE_MASK(64),
+	.mult		= 1,
 };
 
+/* The "scheduler clock" is just our real clock, adjusted to start at zero */
 static unsigned long long lguest_sched_clock(void)
 {
 	return cyc2ns(&lguest_clock, lguest_clock_read() - clock_base);
@@ -742,24 +766,21 @@ static void lguest_time_init(void)
 	set_irq_handler(0, lguest_time_irq);
 
 	/* Our clock structure look like arch/i386/kernel/tsc.c if we can use
-	 * the TSC, otherwise it looks like kernel/time/jiffies.c.  Either way,
-	 * the "rating" is initialized so high that it's always chosen over any
-	 * other clocksource. */
+	 * the TSC, otherwise it's a dumb nanosecond-resolution clock.  Either
+	 * way, the "rating" is initialized so high that it's always chosen
+	 * over any other clocksource. */
 	if (lguest_data.tsc_khz) {
 		lguest_clock.shift = 22;
 		lguest_clock.mult = clocksource_khz2mult(lguest_data.tsc_khz,
 							 lguest_clock.shift);
-		lguest_clock.mask = CLOCKSOURCE_MASK(64);
 		lguest_clock.flags = CLOCK_SOURCE_IS_CONTINUOUS;
-	} else {
-		/* To understand this, start at kernel/time/jiffies.c... */
-		lguest_clock.shift = 8;
-		lguest_clock.mult = (((u64)NSEC_PER_SEC<<8)/ACTHZ) << 8;
-		lguest_clock.mask = CLOCKSOURCE_MASK(32);
 	}
 	clock_base = lguest_clock_read();
 	clocksource_register(&lguest_clock);
 
+	/* Now we've set up our clock, we can use it as the scheduler clock */
+	paravirt_ops.sched_clock = lguest_sched_clock;
+
 	/* We can't set cpumask in the initializer: damn C limitations!  Set it
 	 * here and register our timer device. */
 	lguest_clockevent.cpumask = cpumask_of_cpu(0);
@@ -996,7 +1017,6 @@ __init void lguest_init(void *boot)
 	paravirt_ops.time_init = lguest_time_init;
 	paravirt_ops.set_lazy_mode = lguest_lazy_mode;
 	paravirt_ops.wbinvd = lguest_wbinvd;
-	paravirt_ops.sched_clock = lguest_sched_clock;
 	/* Now is a good time to look at the implementations of these functions
 	 * before returning to the rest of lguest_init(). */