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SubjectRe: [PATCH] kvm: fix i8254 counter 0 wraparound
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@csail.mit.edu>
Applied, thanks!

> ---
> 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.


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