From: Nickolai Zeldovich <nickolai@csail.mit.edu>
Date: Sat, 15 Dec 2012 11:34:37 +0000 (-0500)
Subject: kvm: fix i8254 counter 0 wraparound
X-Git-Tag: v3.9-rc1~97^2~91
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d4b06c2d4cce466e2d62163c0a954e1b2ce96f8b;p=pandora-kernel.git

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>
---

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