On Sat, Dec 15, 2012 at 06:34:37AM -0500, Nickolai Zeldovich wrote: > The kvm i8254 emulation for counter 0 (but not for counters 1 and 2) > has at least two bugs in mode 0: > > 1. The OUT bit, computed by pit_get_out(), is never set high. > > 2. The counter value, computed by pit_get_count(), wraps back around to > the initial counter value, rather than wrapping back to 0xFFFF > (which is the behavior described in the comment in __kpit_elapsed, > the behavior implemented by qemu, and the behavior observed on AMD > hardware). > > The bug stems from __kpit_elapsed computing the elapsed time mod the > initial counter value (stored as nanoseconds in ps->period). This is both > unnecessary (none of the callers of kpit_elapsed expect the value to be > at most the initial counter value) and incorrect (it causes pit_get_count > to appear to wrap around to the initial counter value rather than 0xFFFF). > Removing this mod from __kpit_elapsed fixes both of the above bugs. > > Signed-off-by: Nickolai Zeldovich <nickolai@xxxxxxxxxxxxx> Looks good to me. Marcelo, you were last touching the code. Can you look at it too? > --- > arch/x86/kvm/i8254.c | 1 - > 1 file changed, 1 deletion(-) > > diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c > index 11300d2..c1d30b2 100644 > --- a/arch/x86/kvm/i8254.c > +++ b/arch/x86/kvm/i8254.c > @@ -122,7 +122,6 @@ static s64 __kpit_elapsed(struct kvm *kvm) > */ > remaining = hrtimer_get_remaining(&ps->timer); > elapsed = ps->period - ktime_to_ns(remaining); > - elapsed = mod_64(elapsed, ps->period); > > return elapsed; > } > -- > 1.7.10.4 -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
