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

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Date:	Wed, 04 Mar 2015 15:41:07 +0100
From:	Juergen Gross <jgross@...e.com>
To:	Ian Campbell <ian.campbell@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	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 03/04/2015 03:29 PM, Ian Campbell wrote:
> On Wed, 2015-03-04 at 14:19 +0000, David Vrabel wrote:
>> On 04/03/15 14:09, Juergen Gross wrote:
>>>
>>> The main question whether it is worth to consider this alternative is
>>> the performance aspect. Does anyone have an idea which USB devices would
>>> typically be used via pvusb? I'd suspect memory sticks and USB disks
>>> and perhaps webcams being the most performance relevant ones. Is an
>>> additional copy operation of user data acceptable here?
>>
>> I have no idea.  We (XenServer) have no use cases at all for USB device
>> passthrough.
>
> My gut feeling is that for USB 1 and 2 the bus itself isn't fast enough
> that anyone would care. qdisk has acceptable for disks, so it's probably
> ok for usb too.

While I can accept the bus speed reasoning, I doubt qdisk is copying
data between user and kernel space under normal circumstances. I think
disk I/Os are done using DMA to/from the user buffer directly.

> For usb 3 onwards, well, maybe when we care about those we'll decide
> that a kernel space driver is needed, but for now it seems like
> userspace would be ok.

Do you have another feeling about the probability of a need to do usb 3?
If it is already on the horizon I wouldn't want to do the user space
backend now and the kernel one next year. :-)


Juergen
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