From: Andriy Gapon <avg@FreeBSD.org>
To: qemu-devel@nongnu.org
Subject: [Qemu-devel] usb-ohci: td.cbp incorrectly updated near page end
Date: Thu, 22 Dec 2011 11:34:30 +0200 [thread overview]
Message-ID: <4EF2F9A6.3020601@FreeBSD.org> (raw)
The current code that updates the cbp value after a transfer looks like this:
td.cbp += ret;
if ((td.cbp & 0xfff) + ret > 0xfff) {
<handle page overflow>
because the 'ret' value is effectively added twice the check may fire too early
when the overflow hasn't happened yet.
Below is one of the possible changes that correct the behavior:
--- hw/usb-ohci.c.orig 2011-12-22 02:44:49.650537164 +0200
+++ hw/usb-ohci.c 2011-12-22 03:50:37.545642734 +0200
@@ -1025,10 +1031,10 @@ static int ohci_service_td(OHCIState *oh
if (ret == len) {
td.cbp = 0;
} else {
- td.cbp += ret;
if ((td.cbp & 0xfff) + ret > 0xfff) {
- td.cbp &= 0xfff;
- td.cbp |= td.be & ~0xfff;
+ td.cbp = (td.be & ~0xfff) + ((td.cbp + ret) & 0xfff);
+ } else {
+ td.cbp += ret;
}
}
td.flags |= OHCI_TD_T1;
--
Andriy Gapon
next reply other threads:[~2011-12-22 9:45 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-22 9:34 Andriy Gapon [this message]
2011-12-30 11:07 ` [Qemu-devel] usb-ohci: td.cbp incorrectly updated near page end Stefan Hajnoczi
2012-01-06 9:47 ` Gerd Hoffmann
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