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authorNickolai Zeldovich <nickolai@csail.mit.edu>2012-12-15 06:34:37 -0500
committerGleb Natapov <gleb@redhat.com>2012-12-18 11:12:38 +0200
commitd4b06c2d4cce466e2d62163c0a954e1b2ce96f8b (patch)
treea8bc53f6855a6fa8208ef6544c36222febfb0d1e
parente11ae1a102b46f76441e328a2743ae5d6e201423 (diff)
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>
-rw-r--r--arch/x86/kvm/i8254.c1
1 files changed, 0 insertions, 1 deletions
diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
index 11300d2fa71..c1d30b2fc9b 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;
}