From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.4 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,T_HK_NAME_DR,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8461BC04A6B for ; Fri, 10 May 2019 09:36:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 623DA20881 for ; Fri, 10 May 2019 09:36:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727504AbfEJJg1 (ORCPT ); Fri, 10 May 2019 05:36:27 -0400 Received: from mx1.redhat.com ([209.132.183.28]:51530 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727136AbfEJJgZ (ORCPT ); Fri, 10 May 2019 05:36:25 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D5BCE308FC20; Fri, 10 May 2019 09:36:24 +0000 (UTC) Received: from work-vm (ovpn-117-163.ams2.redhat.com [10.36.117.163]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 806C858B1; Fri, 10 May 2019 09:36:11 +0000 (UTC) Date: Fri, 10 May 2019 10:36:09 +0100 From: "Dr. David Alan Gilbert" To: Cornelia Huck Cc: Alex Williamson , Yan Zhao , intel-gvt-dev@lists.freedesktop.org, arei.gonglei@huawei.com, aik@ozlabs.ru, Zhengxiao.zx@alibaba-inc.com, shuangtai.tst@alibaba-inc.com, qemu-devel@nongnu.org, eauger@redhat.com, yi.l.liu@intel.com, ziye.yang@intel.com, mlevitsk@redhat.com, pasic@linux.ibm.com, felipe@nutanix.com, changpeng.liu@intel.com, Ken.Xue@amd.com, jonathan.davies@nutanix.com, shaopeng.he@intel.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, libvir-list@redhat.com, eskultet@redhat.com, kevin.tian@intel.com, zhenyuw@linux.intel.com, zhi.a.wang@intel.com, cjia@nvidia.com, kwankhede@nvidia.com, berrange@redhat.com, dinechin@redhat.com Subject: Re: [PATCH v2 1/2] vfio/mdev: add version attribute for mdev device Message-ID: <20190510093608.GD2854@work-vm> References: <20190506014514.3555-1-yan.y.zhao@intel.com> <20190506014904.3621-1-yan.y.zhao@intel.com> <20190507151826.502be009@x1.home> <20190509173839.2b9b2b46.cohuck@redhat.com> <20190509154857.GF2868@work-vm> <20190509175404.512ae7aa.cohuck@redhat.com> <20190509164825.GG2868@work-vm> <20190510110838.2df4c4d0.cohuck@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190510110838.2df4c4d0.cohuck@redhat.com> User-Agent: Mutt/1.11.4 (2019-03-13) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.43]); Fri, 10 May 2019 09:36:25 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Cornelia Huck (cohuck@redhat.com) wrote: > On Thu, 9 May 2019 17:48:26 +0100 > "Dr. David Alan Gilbert" wrote: > > > * Cornelia Huck (cohuck@redhat.com) wrote: > > > On Thu, 9 May 2019 16:48:57 +0100 > > > "Dr. David Alan Gilbert" wrote: > > > > > > > * Cornelia Huck (cohuck@redhat.com) wrote: > > > > > On Tue, 7 May 2019 15:18:26 -0600 > > > > > Alex Williamson wrote: > > > > > > > > > > > On Sun, 5 May 2019 21:49:04 -0400 > > > > > > Yan Zhao wrote: > > > > > > > > > > > > + Errno: > > > > > > > + If vendor driver wants to claim a mdev device incompatible to all other mdev > > > > > > > + devices, it should not register version attribute for this mdev device. But if > > > > > > > + a vendor driver has already registered version attribute and it wants to claim > > > > > > > + a mdev device incompatible to all other mdev devices, it needs to return > > > > > > > + -ENODEV on access to this mdev device's version attribute. > > > > > > > + If a mdev device is only incompatible to certain mdev devices, write of > > > > > > > + incompatible mdev devices's version strings to its version attribute should > > > > > > > + return -EINVAL; > > > > > > > > > > > > I think it's best not to define the specific errno returned for a > > > > > > specific situation, let the vendor driver decide, userspace simply > > > > > > needs to know that an errno on read indicates the device does not > > > > > > support migration version comparison and that an errno on write > > > > > > indicates the devices are incompatible or the target doesn't support > > > > > > migration versions. > > > > > > > > > > I think I have to disagree here: It's probably valuable to have an > > > > > agreed error for 'cannot migrate at all' vs 'cannot migrate between > > > > > those two particular devices'. Userspace might want to do different > > > > > things (e.g. trying with different device pairs). > > > > > > > > Trying to stuff these things down an errno seems a bad idea; we can't > > > > get much information that way. > > > > > > So, what would be a reasonable approach? Userspace should first read > > > the version attributes on both devices (to find out whether migration > > > is supported at all), and only then figure out via writing whether they > > > are compatible? > > > > > > (Or just go ahead and try, if it does not care about the reason.) > > > > Well, I'm OK with something like writing to test whether it's > > compatible, it's just we need a better way of saying 'no'. > > I'm not sure if that involves reading back from somewhere after > > the write or what. > > Hm, so I basically see two ways of doing that: > - standardize on some error codes... problem: error codes can be hard > to fit to reasons > - make the error available in some attribute that can be read > > I'm not sure how we can serialize the readback with the last write, > though (this looks inherently racy). > > How important is detailed error reporting here? I think we need something, otherwise we're just going to get vague user reports of 'but my VM doesn't migrate'; I'd like the error to be good enough to point most users to something they can understand (e.g. wrong card family/too old a driver etc). Dave -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK