1 files changed, 36 insertions, 16 deletions

diff --git a/drivers/lguest/lguest.c b/drivers/lguest/lguest.c
index 3386b0e76900..1bc1546c7fd0 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(). */