linux-kernel - Re: [Xen-devel] [PATCH 3/4] usb: Introduce Xen pvUSB backend

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Date:	Wed, 04 Mar 2015 14:31:08 +0100
From:	Juergen Gross <jgross@...e.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com,
	linux-usb@...r.kernel.org, gregkh@...uxfoundation.org,
	cyliu@...e.com
Subject: Re: [Xen-devel] [PATCH 3/4] usb: Introduce Xen pvUSB backend

On 03/02/2015 12:39 PM, David Vrabel wrote:
> On 26/02/15 13:35, Juergen Gross wrote:
>> Introduces the Xen pvUSB backend. With pvUSB it is possible for a Xen
>> domU to communicate with a USB device assigned to that domU. The
>> communication is all done via the pvUSB backend in a driver domain
>> (usually Dom0) which is owner of the physical device.
>
> Why do we need a kernel usb backend instead of a user-space one using
> libusb?

Good question. At a first glance libusb seems to offer most/all needed
interfaces. The main question is whether performance with libusb will
be okay. There will be one additional copy of the I/O data needed if
I've read the code in drivers/usb/core/devio.c correctly.

I haven't worked with user space backends yet. Do you recommend a
special example I should start with?

>
>> Changes from the original version are:
>> - port to upstream kernel
>> - put all code in just one source file
>
> ?? I'm not sure that was an improvement.  The resulting single file is
> too large IMO.

OTOH this reduces overall code size:

New file has 1845 lines, while old version had in sum 2243 lines with
the largest file having 1217 lines. So the largest file is 50% larger,
while overall size is 20% smaller.

>> - move module to appropriate location in kernel tree
>
> drivers/xen/ is the correct location for this driver.

Hmm, so you regard placement of xen-netback under drivers/net and
xen-blkback under drivers/block as wrong? I've just followed these
examples.

Juergen
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