Re: [Qemu-devel] Fwd: [PATCH v2] Guest OS hangs on usb_add

[TJ <one.timothy.jones@gmail.com>]

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