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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=unavailable 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 7D638C04A6B for ; Mon, 13 May 2019 01:23:13 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 42526208CB for ; Mon, 13 May 2019 01:23:13 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 42526208CB Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([127.0.0.1]:49290 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hPzgO-0006Z8-Do for qemu-devel@archiver.kernel.org; Sun, 12 May 2019 21:23:12 -0400 Received: from eggs.gnu.org ([209.51.188.92]:54174) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hPzfY-0006Fx-7H for qemu-devel@nongnu.org; Sun, 12 May 2019 21:22:21 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hPzfW-00008m-QJ for qemu-devel@nongnu.org; Sun, 12 May 2019 21:22:20 -0400 Received: from mga01.intel.com ([192.55.52.88]:37896) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hPzfW-00005f-Gg for qemu-devel@nongnu.org; Sun, 12 May 2019 21:22:18 -0400 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by fmsmga101.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 12 May 2019 18:22:09 -0700 X-ExtLoop1: 1 Received: from joy-optiplex-7040.sh.intel.com (HELO joy-OptiPlex-7040) ([10.239.13.9]) by fmsmga005.fm.intel.com with ESMTP; 12 May 2019 18:22:05 -0700 Date: Sun, 12 May 2019 21:16:26 -0400 From: Yan Zhao To: Cornelia Huck Message-ID: <20190513011626.GI24397@joy-OptiPlex-7040> 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> <20190510093608.GD2854@work-vm> <20190510114838.7e16c3d6.cohuck@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190510114838.7e16c3d6.cohuck@redhat.com> User-Agent: Mutt/1.9.4 (2018-02-28) X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-Received-From: 192.55.52.88 Subject: Re: [Qemu-devel] [PATCH v2 1/2] vfio/mdev: add version attribute for mdev device X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Yan Zhao Cc: "cjia@nvidia.com" , "kvm@vger.kernel.org" , "aik@ozlabs.ru" , "Zhengxiao.zx@alibaba-inc.com" , "shuangtai.tst@alibaba-inc.com" , "qemu-devel@nongnu.org" , "kwankhede@nvidia.com" , "eauger@redhat.com" , "Liu, Yi L" , "eskultet@redhat.com" , "Yang, Ziye" , "mlevitsk@redhat.com" , "pasic@linux.ibm.com" , "libvir-list@redhat.com" , "arei.gonglei@huawei.com" , "felipe@nutanix.com" , "Ken.Xue@amd.com" , "Tian, Kevin" , "Dr. David Alan Gilbert" , "zhenyuw@linux.intel.com" , "dinechin@redhat.com" , Alex Williamson , "intel-gvt-dev@lists.freedesktop.org" , "Liu, Changpeng" , "berrange@redhat.com" , "linux-kernel@vger.kernel.org" , "Wang, Zhi A" , "jonathan.davies@nutanix.com" , "He, Shaopeng" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, May 10, 2019 at 05:48:38PM +0800, Cornelia Huck wrote: > On Fri, 10 May 2019 10:36:09 +0100 > "Dr. David Alan Gilbert" wrote: > > > * 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). > > Ok, that sounds like a reasonable point. Not that I have a better idea > how to achieve that, though... we could also log a more verbose error > message to the kernel log, but that's not necessarily where a user will > look first. > > Ideally, we'd want to have the user space program setting up things > querying the general compatibility for migration (so that it becomes > their problem on how to alert the user to problems :), but I'm not sure > how to eliminate the race between asking the vendor driver for > compatibility and getting the result of that operation. > > Unless we introduce an interface that can retrieve _all_ results > together with the written value? Or is that not going to be much of a > problem in practice? what about defining a migration_errors attribute, storing recent 10 error records with format like: input string: error as identical input strings always have the same error string, the 10 error records may meet 10+ reason querying operations. And in practice, I think there wouldn't be 10 simultaneous migration requests? or could we just define some common errno? like #define ENOMIGRATION 140 /* device not supporting migration */ #define EUNATCH 49 /* software version not match */ #define EHWNM 142 /* hardware not matching*/