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Re: [Xen-devel] XEN CD --> USB stick

> > The Linux USB stack seems to work OK.  I never convinced
> > myself that the
> > default Linux HCD drivers really understood pseudophysical
> > memory but it
> > doesn't seem to have bitten me in practice *touch wood*
> All the pseudophysical memory issues shoud be taken care of within the
> Linux macros. It's only if the driver is buggy that its an issue.

Take a look at 2.6/drivers/usb/host/uhci-hcd.c::uhci_submit_common.  It 
doesn't do the conversions required to cope with pseudophysical memory.  As 
far as I can tell, USB client drivers cheerfully pass kmalloc'd memory down 
to this function.

As far as I'm concerned, this is still a worry.

> > usbfront / usbback need more development to support all sorts
> > of random
> > devices, so although it's stable and functional for me right
> > now I don't
> > think this should be enabled by default.
> OK, thanks, that's good to know.
> Has usbfront/back been forwarded ported to work on 2.6 yet?
> Is there stuff we could do to make it more stable, perhaps restricting
> the set of devices it attempts to virtualize?

Harry's being doing some good work on the 2.6 port but it's not completed yet.

Devices that don't virtualise well just don't work, it doesn't impact 
stability.  I doubt there's any way of identifying these devices other than 
by trying them out (depends whether the manufacturer put a stupid chip in the 

Eventually we will support basically everything but that'll require faking out 
much more in software, which is probably not worth doing yet.

Bottom line: people who want to play with USB virtualisation should be able to 
do so successfully but should bear in mind it's experimental.


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