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

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Message-ID: <20150306105944.GK32543@reaktio.net>
Date:	Fri, 6 Mar 2015 12:59:44 +0200
From:	Pasi Kärkkäinen <pasik@....fi>
To:	Ian Campbell <ian.campbell@...rix.com>
Cc:	Juergen Gross <jgross@...e.com>, xen-devel@...ts.xensource.com,
	gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, cyliu@...e.com,
	David Vrabel <david.vrabel@...rix.com>,
	boris.ostrovsky@...cle.com
Subject: Re: [Xen-devel] [PATCH 3/4] usb: Introduce Xen pvUSB backend

On Wed, Mar 04, 2015 at 02:46:33PM +0000, Ian Campbell wrote:
> On Wed, 2015-03-04 at 15:41 +0100, Juergen Gross wrote:
> > 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.
> > >

I've seen XenServer customers asking for USB passthru functionality on IRC channels..


> > > 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.
> 
> I thought there was at least one copy on the datapath with qdisk,
> wherever it is. But I don't know for sure.
> 
> > > 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. :-)
> 
> Well, what is *your* use case for USB passthru? I don't actually have
> one myself.
> 
> I'd speculate that people are more interested in passing in
> low/medium/high speed devices rather than the superfast usb3 disks etc.
> But I have no reason to back that up.
> 

People seem to ask/need USB passthru support for various use cases 
including passing thru: webcams, video grabbers, printers, mobile broadband modems, 
copy protection dongles, gaming controllers, you name it..


-- Pasi

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