From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758582AbbCDOqi (ORCPT ); Wed, 4 Mar 2015 09:46:38 -0500 Received: from smtp.citrix.com ([66.165.176.89]:50587 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758420AbbCDOqg (ORCPT ); Wed, 4 Mar 2015 09:46:36 -0500 X-IronPort-AV: E=Sophos;i="5.09,687,1418083200"; d="scan'208";a="239817757" Message-ID: <1425480393.25940.181.camel@citrix.com> Subject: Re: [Xen-devel] [PATCH 3/4] usb: Introduce Xen pvUSB backend From: Ian Campbell To: Juergen Gross CC: David Vrabel , , , , , , , Date: Wed, 4 Mar 2015 14:46:33 +0000 In-Reply-To: <54F71983.6070708@suse.com> 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> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.9-1+b1 MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-DLP: MIA1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > > > 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. Ian. From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ian Campbell Subject: Re: [Xen-devel] [PATCH 3/4] usb: Introduce Xen pvUSB backend Date: Wed, 4 Mar 2015 14:46:33 +0000 Message-ID: <1425480393.25940.181.camel@citrix.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <54F71983.6070708@suse.com> Sender: linux-kernel-owner@vger.kernel.org To: Juergen Gross Cc: David Vrabel , linux-kernel@vger.kernel.org, xen-devel@lists.xensource.com, konrad.wilk@oracle.com, boris.ostrovsky@oracle.com, linux-usb@vger.kernel.org, gregkh@linuxfoundation.org, cyliu@suse.com List-Id: xen-devel@lists.xenproject.org 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. > > > > 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. Ian.