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author | Nickolai Zeldovich <nickolai@csail.mit.edu> | 2012-12-15 11:34:37 (GMT) |
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committer | Gleb Natapov <gleb@redhat.com> | 2012-12-18 09:12:38 (GMT) |
commit | d4b06c2d4cce466e2d62163c0a954e1b2ce96f8b (patch) | |
tree | a8bc53f6855a6fa8208ef6544c36222febfb0d1e /arch/x86/kvm | |
parent | e11ae1a102b46f76441e328a2743ae5d6e201423 (diff) | |
download | linux-fsl-qoriq-d4b06c2d4cce466e2d62163c0a954e1b2ce96f8b.tar.xz |
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>
Diffstat (limited to 'arch/x86/kvm')
-rw-r--r-- | arch/x86/kvm/i8254.c | 1 |
1 files changed, 0 insertions, 1 deletions
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; } |