[Spice-devel] [RFC PATCH 1/1] Add a usbredir kernel module to remotely connect USB devices over IP.

[Alan Stern]

On Fri, 3 Jul 2015, Krzysztof Opasiak wrote:

> > The point is that a device driver like usbip _cannot_ isolate the
> > running kernel from the vagaries of the network transport if part of
> > that transport occurs in userspace.
> >
> > If any part of the transport passes through userspace, you can end up
> > in a situation like what I outlined above, where a message can't be
> > transported until after its reply has been received.  There's no way
> > for a device driver to prevent a deadlock when this occurs, no matter
> > what it virtualizes.
> >
> 
> Doesn't we have the same problem with functionfs/gadgetfs and dummy_hcd? 
> Or with fuse?

Yes indeed.

> It's a very generic problem for all "virtualized devices" and it is 
> known for quite a long time. This is why many tutorials about swap warns 
> that swap should be set up only on real block devices which are fully 
> served in kernel.

That is good advice.  :-)

Alan Stern