KVM: PIT: fix count read and mode 0 handling

Marcelo Tosatti
2009-04-08T16:16:06Z

Commit 46ee278652f4cbd51013471b64c7897ba9bcd1b1 causes Solaris 10
to hang on boot.

Assuming that PIT counter reads should return 0 for an expired timer
is wrong: when it is active, the counter never stops (see comment on
__kpit_elapsed).

Also arm a one shot timer for mode 0.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Index: kvm/arch/x86/kvm/i8254.c
===================================================================
--- kvm.orig/arch/x86/kvm/i8254.c
+++ kvm/arch/x86/kvm/i8254.c
@@ -104,13 +104,18 @@ static s64 __kpit_elapsed(struct kvm *kv
ktime_t remaining;
struct kvm_kpit_state *ps = &kvm->arch.vpit->pit_state;

+ /*
+ * The Counter does not stop when it reaches zero. In
+ * Modes 0, 1, 4, and 5 the Counter ``wraps around'' to
+ * the highest count, either FFFF hex for binary counting
+ * or 9999 for BCD counting, and continues counting.
+ * Modes 2 and 3 are periodic; the Counter reloads
+ * itself with the initial count and continues counting
+ * from there.
+ */
remaining = hrtimer_expires_remaining(&ps->pit_timer.timer);
- if (ktime_to_ns(remaining) < 0)
- remaining = ktime_set(0, 0);
-
- elapsed = ps->pit_timer.period;
- if (ktime_to_ns(remaining) <= ps->pit_timer.period)
- elapsed = ps->pit_timer.period - ktime_to_ns(remaining);
+ elapsed = ps->pit_timer.period - ktime_to_ns(remaining);
+ elapsed = mod_64(elapsed, ps->pit_timer.period);

return elapsed;
}
@@ -280,7 +285,7 @@ static void create_pit_timer(struct kvm_

/* TODO The new value only affected after the retriggered */
hrtimer_cancel(&pt->timer);
- pt->period = (is_period == 0) ? 0 : interval;
+ pt->period = interval;
ps->is_periodic = is_period;

pt->timer.function = kvm_timer_fn;
@@ -304,10 +309,8 @@ static void pit_load_count(struct kvm *k
pr_debug("pit: load_count val is %d, channel is %d\n", val, channel);

/*
- * Though spec said the state of 8254 is undefined after power-up,
- * seems some tricky OS like Windows XP depends on IRQ0 interrupt
- * when booting up.
- * So here setting initialize rate for it, and not a specific number
+ * The largest possible initial count is 0; this is equivalent
+ * to 216 for binary counting and 104 for BCD counting.
*/
if (val == 0)
val = 0x10000;
@@ -322,6 +325,7 @@ static void pit_load_count(struct kvm *k
/* Two types of timer
* mode 1 is one shot, mode 2 is period, otherwise del timer */
switch (ps->channels[0].mode) {
+ case 0:
case 1:
/* FIXME: enhance mode 4 precision */
case 4: