From d4b06c2d4cce466e2d62163c0a954e1b2ce96f8b Mon Sep 17 00:00:00 2001 From: Nickolai Zeldovich <nickolai@csail.mit.edu> Date: Sat, 15 Dec 2012 06:34:37 -0500 Subject: [PATCH] kvm: fix i8254 counter 0 wraparound 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> Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com> Signed-off-by: Gleb Natapov <gleb@redhat.com> --- 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 11300d2fa7144..c1d30b2fc9bb1 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; } -- 2.30.2