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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 C0384C33C8C for ; Tue, 7 Jan 2020 17:50:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 952E121744 for ; Tue, 7 Jan 2020 17:50:55 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="WrTtwqVg" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728429AbgAGRuy (ORCPT ); Tue, 7 Jan 2020 12:50:54 -0500 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:54287 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728348AbgAGRuy (ORCPT ); Tue, 7 Jan 2020 12:50:54 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1578419453; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=2GTm1l5zQtZ52XhEMdXoYtz6Pjm3g5KvI/aIwxC4+9w=; b=WrTtwqVgPVINw4HqHyHyhLSL53diSxaqqmCWifSF7rAzv1hSvoIal3XZpU96oTub02S5Am V0e82ze+2RlzzFweorsviwpE6BpC131Ried7W2osSDNnG+Dvqmol7k3Kc0Wr+7Ev/eBXc+ neUeBojm2N6bysjWwJb13cdvb+c3Ak4= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-10-cWjUiVAuMqeB90ek5Knc5w-1; Tue, 07 Jan 2020 12:50:49 -0500 X-MC-Unique: cWjUiVAuMqeB90ek5Knc5w-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 2577E1951287; Tue, 7 Jan 2020 17:50:47 +0000 (UTC) Received: from work-vm (ovpn-117-52.ams2.redhat.com [10.36.117.52]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E77B29A84; Tue, 7 Jan 2020 17:50:40 +0000 (UTC) Date: Tue, 7 Jan 2020 17:50:37 +0000 From: "Dr. David Alan Gilbert" To: Alex Williamson Cc: Kirti Wankhede , cjia@nvidia.com, kevin.tian@intel.com, ziye.yang@intel.com, changpeng.liu@intel.com, yi.l.liu@intel.com, mlevitsk@redhat.com, eskultet@redhat.com, cohuck@redhat.com, jonathan.davies@nutanix.com, eauger@redhat.com, aik@ozlabs.ru, pasic@linux.ibm.com, felipe@nutanix.com, Zhengxiao.zx@alibaba-inc.com, shuangtai.tst@alibaba-inc.com, Ken.Xue@amd.com, zhi.a.wang@intel.com, yan.y.zhao@intel.com, qemu-devel@nongnu.org, kvm@vger.kernel.org Subject: Re: [PATCH v10 Kernel 1/5] vfio: KABI for migration interface for device state Message-ID: <20200107175037.GL2732@work-vm> References: <20191217114357.6496f748@x1.home> <3527321f-e310-8324-632c-339b22f15de5@nvidia.com> <20191219102706.0a316707@x1.home> <928e41b5-c3fd-ed75-abd6-ada05cda91c9@nvidia.com> <20191219140929.09fa24da@x1.home> <20200102182537.GK2927@work-vm> <20200106161851.07871e28@w520.home> <20200107095740.GB2778@work-vm> <20200107095410.2be5a064@w520.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200107095410.2be5a064@w520.home> User-Agent: Mutt/1.13.0 (2019-11-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org * Alex Williamson (alex.williamson@redhat.com) wrote: > On Tue, 7 Jan 2020 09:57:40 +0000 > "Dr. David Alan Gilbert" wrote: > > > * Alex Williamson (alex.williamson@redhat.com) wrote: > > > On Thu, 2 Jan 2020 18:25:37 +0000 > > > "Dr. David Alan Gilbert" wrote: > > > > > > > * Alex Williamson (alex.williamson@redhat.com) wrote: > > > > > On Fri, 20 Dec 2019 01:40:35 +0530 > > > > > Kirti Wankhede wrote: > > > > > > > > > > > On 12/19/2019 10:57 PM, Alex Williamson wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > If device state it at pre-copy state (011b). > > > > > > Transition, i.e., write to device state as stop-and-copy state (010b) > > > > > > failed, then by previous state I meant device should return pre-copy > > > > > > state(011b), i.e. previous state which was successfully set, or as you > > > > > > said current state which was successfully set. > > > > > > > > > > Yes, the point I'm trying to make is that this version of the spec > > > > > tries to tell the user what they should do upon error according to our > > > > > current interpretation of the QEMU migration protocol. We're not > > > > > defining the QEMU migration protocol, we're defining something that can > > > > > be used in a way to support that protocol. So I think we should be > > > > > concerned with defining our spec, for example my proposal would be: "If > > > > > a state transition fails the user can read device_state to determine the > > > > > current state of the device. This should be the previous state of the > > > > > device unless the vendor driver has encountered an internal error, in > > > > > which case the device may report the invalid device_state 110b. The > > > > > user must use the device reset ioctl in order to recover the device > > > > > from this state. If the device is indicated in a valid device state > > > > > via reading device_state, the user may attempt to transition the device > > > > > to any valid state reachable from the current state." > > > > > > > > We might want to be able to distinguish between: > > > > a) The device has failed and needs a reset > > > > b) The migration has failed > > > > > > I think the above provides this. For Kirti's example above of > > > transitioning from pre-copy to stop-and-copy, the device could refuse > > > to transition to stop-and-copy, generating an error on the write() of > > > device_state. The user re-reading device_state would allow them to > > > determine the current device state, still in pre-copy or failed. Only > > > the latter would require a device reset. > > > > OK - but that doesn't give you any way to figure out 'why' it failed; > > I guess I was expecting you to then read an 'error' register to find > > out what happened. > > Assuming the write() to transition to stop-and-copy fails and you're > > still in pre-copy, what's the defined thing you're supposed to do next? > > Decide migration has failed and then do a write() to transition to running? > > Defining semantics for an error register seems like a project on its > own. We do have flags, we could use them to add an error register > later, but I think it's only going to rat hole this effort to try to > incorporate that now. OK, to be honest I didn't really mean for that thing to be used by code to decide on it's next action, rather to have something to report when it failed. > The state machine is fairly small, so in the > scenario you present, I think the user would assume a failure at > pre-copy to stop-and-copy transition would fail the migration and the > device could go back to running state. If the device then fails to > return to the running state, we might be stuck with a device with > reduced performance or overhead and the user could warn about that and > continue with the device as-is. The vendor drivers could make use of > -EAGAIN on transition failure to indicate a temporary issue, but > otherwise the user should probably consider it a persistent error until > either a device reset or start of a new migration sequence (ie. return > to running and start over). Thanks, OK as long as we define somewhere that the action on a failed transition is then try and transitino to running before restarting the VM and fail the migration. Dave > Alex -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK