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

[Anthony Liguori <anthony@codemonkey.ws>]

On 11/16/2010 01:36 PM, TJ wrote:
> 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?
>    

Yeah, that bit is a bit too gnarly for my tastes, but if you can resend 
the rest of it with a Signed-off-by, I'd appreciate.

Regards,

Anthony Liguori

> -TJ
>