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[Xen-devel] Re: Re: [PATCH] Hook USB... and a first question about the USB code

OK.  Whatever you think is best. 

Finally, I have a question about the code:

The back end driver is calling usb_set_configuration which has disappeared in 
the 2.6 kernel.

My understanding of this after having extracted my head from the blender of the 
USB spec is that the configuration determines what USB resources are used i.e. 
USB bandwidth and power and the reason that this has been removed from the 2.6 
driver interface is that this should be a policy defined by the user via 
hotplug scripts and not set by the driver.

But the USB virtualisation code is, as far as I can tell, letting the guest OS 
instance set the configuration from the front end.

It seems to me that this A) isn't compatible with the new 2.6 interface and B) 
isn't what is required for protection between the guest OSs anyway.

I think the correct design would be for the back-end domain to set the 
configuration based on the Xen guest configuration and then present the usb 
device to the front end as if it only had one configuration and fake up the USB 
device state machine such that bus enumeration could be performed in the guest 
without affecting the back end.

This would allow the USB bus resources to be managed with protection between 
the different guests but would make devices with multiple configurations look 
different in the guests---not sure how this would affect driver compatibility.

Alternatively, there is some provision in the USB spec for declaring that a HCI 
has less bandwidth available than the theoretical USB maximum.  I guess, using 
this mechanism, the available USB B/W might be divided up between the guests 
and then they might be allowed to set configurations within the constraints of 
their allocated bandwidths.  Not too sure about this option though: it still 
seems incompatible with the 2.6 interface and care would be required to ensure 
isolation didn't rely on cooperation from the guests.

What are your thoughts on this?


> Message date : Feb 01 2005, 02:50 PM
> From : "Mark A. Williamson" <mark.williamson@xxxxxxxxxxxx>
> To : "Harry Butterworth" <harry@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
> Copy to : 
> Subject : Re: [PATCH] Hook USB virt code into 2.6 build
> Thanks Harry,
> I'll take a look at this but I'm in two minds whether to merge it just yet.  
> One the one hand, it seems like a reasonable thing to do.  On the other hand, 
> people might see the option there and wonder why it breaks the build...
> Cheers,
> Mark
> On Monday 31 January 2005 18:43, you wrote:
> > This patch creates the Makefiles and the Kconfig but doesn't fix the
> > compile issues.  The config options are set to n to avoid breaking the
> > 2.6 build. Tested with make world.


Whatever you Wanadoo:

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