From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=55528 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PIRLJ-0003lC-Sp for qemu-devel@nongnu.org; Tue, 16 Nov 2010 14:37:30 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PIRLI-0003p6-KG for qemu-devel@nongnu.org; Tue, 16 Nov 2010 14:37:25 -0500 Received: from mail-yx0-f173.google.com ([209.85.213.173]:40551) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PIRLI-0003p0-CY for qemu-devel@nongnu.org; Tue, 16 Nov 2010 14:37:24 -0500 Received: by yxf34 with SMTP id 34so244622yxf.4 for ; Tue, 16 Nov 2010 11:37:23 -0800 (PST) Message-ID: <4CE2DD27.3020608@gmail.com> Date: Tue, 16 Nov 2010 14:36:07 -0500 From: TJ MIME-Version: 1.0 Subject: Re: [Qemu-devel] Fwd: [PATCH v2] Guest OS hangs on usb_add References: <4CD02560.9030103@gmail.com> <4CE29C77.9080502@codemonkey.ws> In-Reply-To: <4CE29C77.9080502@codemonkey.ws> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: "qemu-devel@nongnu.org" On 11/16/2010 10:00 AM, Anthony Liguori wrote: > On 11/02/2010 09:51 AM, TJ wrote: >> Doesn't look like this has ever been committed. qemu-kvm-0.13 has just arrived >> to the portage tree, but I am still having problems with it. I checked the git >> log and it's not there! Please commit. >> > > One off device hacks are concerning because it's basically impossible to review. > > Why does this work on bare metal? > > Regards, > > Anthony Liguori > Probably because bare metal USB 2.0 controllers don't give a damn about USB 3 spec. :) My guess is that they ignore the device descriptor length and assume that it's always equal 18. Although the USB 2.0 spec doesn't explicitly say anywhere that it can't be more than 18. IIRC USB 3 even adds some extensions to the device descriptor. And since I wanted my code to be portable and USB 3 ready ;) I rely on the value in dev_descr_len. BTW, this patch is more than just a hack for the device in question. Without this patch qemu simply locks up when I attach the remote and spins in endless loop, because USB parsing is so very primitive. With this patch, USB parsing is done more intelligently and devices with whacky USB descriptors are simply rejected. The hack part is really just 3 lines: >> + if (dev_descr_len == 0x18 && dev->descr[ 8] == 0x47 && dev->descr[ 9] == 0x46 >> + && dev->descr[10] == 0x00 && dev->descr[11] == 0x30) >> + dev_descr_len = USB_DT_DEVICE_LEN; /* for buggy MX-950 remote reporting len in hex */ And it is very harmless, as all it does is overwrites the device descriptor length with correct one. If you don't like the hack, you can just remove the 3 lines above and use the rest of the patch. I will just have to remember to manually patch mine every time I upgrade. Your thoughts? -TJ