From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756210AbbCFLJ2 (ORCPT ); Fri, 6 Mar 2015 06:09:28 -0500 Received: from emh01.mail.saunalahti.fi ([62.142.5.107]:45717 "EHLO emh01.mail.saunalahti.fi" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751118AbbCFLJX (ORCPT ); Fri, 6 Mar 2015 06:09:23 -0500 X-Greylist: delayed 575 seconds by postgrey-1.27 at vger.kernel.org; Fri, 06 Mar 2015 06:09:22 EST Date: Fri, 6 Mar 2015 12:59:44 +0200 From: Pasi =?iso-8859-1?Q?K=E4rkk=E4inen?= To: Ian Campbell Cc: Juergen Gross , xen-devel@lists.xensource.com, gregkh@linuxfoundation.org, linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, cyliu@suse.com, David Vrabel , boris.ostrovsky@oracle.com Subject: Re: [Xen-devel] [PATCH 3/4] usb: Introduce Xen pvUSB backend Message-ID: <20150306105944.GK32543@reaktio.net> References: <1424957717-392-1-git-send-email-jgross@suse.com> <1424957717-392-4-git-send-email-jgross@suse.com> <54F44BD5.1030008@citrix.com> <54F7091C.1050001@suse.com> <54F70E3D.20201@citrix.com> <54F71221.6050800@suse.com> <54F71468.3030207@citrix.com> <1425479350.25940.177.camel@citrix.com> <54F71983.6070708@suse.com> <1425480393.25940.181.camel@citrix.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1425480393.25940.181.camel@citrix.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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