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 Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5E1E1C3DA70 for ; Tue, 30 Jul 2024 18:47:29 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1sYrrm-0006rK-Ry; Tue, 30 Jul 2024 14:46:34 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1sYrrl-0006pi-Dm for qemu-devel@nongnu.org; Tue, 30 Jul 2024 14:46:33 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1sYrri-0004cG-NP for qemu-devel@nongnu.org; Tue, 30 Jul 2024 14:46:33 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1722365189; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=yhdkLaCFVE43vsYLjFgMcHHxYaz1sdB8Cy44Q/hw71M=; b=HYl8UbdZq6wNEZnHhwwEVDyAzxAyIUUr9HLpl6HHUqMK2TLYcIeaFNBkbhGQXM/fGyWL7o S+jfzTw4tlnYyH4aG28z5A/8eayxDIhFnpcMUy3mkvuy5NZ3T1SXJveamSa9UR1HFikYWl qpIFeeN63yBL7qNJyCjXrRGkGfPWYqY= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-57-75qLKNDgPsCiQyML4joC-w-1; Tue, 30 Jul 2024 14:46:26 -0400 X-MC-Unique: 75qLKNDgPsCiQyML4joC-w-1 Received: from mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.15]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6F17B1955BED; Tue, 30 Jul 2024 18:46:22 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.108]) by mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id D83301955D42; Tue, 30 Jul 2024 18:46:15 +0000 (UTC) Date: Tue, 30 Jul 2024 19:46:12 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: Thomas Huth , "Michael S. Tsirkin" , Yuri Benditovich , eduardo@habkost.net, marcel.apfelbaum@gmail.com, philmd@linaro.org, wangyanan55@huawei.com, dmitry.fleytman@gmail.com, akihiko.odaki@daynix.com, jasowang@redhat.com, sriram.yagnaraman@est.tech, sw@weilnetz.de, qemu-devel@nongnu.org, yan@daynix.com, Fabiano Rosas , devel@lists.libvirt.org Subject: Re: [PATCH v2 4/4] virtio-net: Add support for USO features Message-ID: References: <20240726020656-mutt-send-email-mst@kernel.org> <775ff713-f7d3-4fdc-8ba0-4ebde577040d@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.12 (2023-09-09) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.15 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.125, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Jul 30, 2024 at 02:13:51PM -0400, Peter Xu wrote: > On Mon, Jul 29, 2024 at 06:26:41PM +0100, Daniel P. Berrangé wrote: > > On Mon, Jul 29, 2024 at 01:00:30PM -0400, Peter Xu wrote: > > > On Mon, Jul 29, 2024 at 04:58:03PM +0100, Daniel P. Berrangé wrote: > > > > > > > > We've got two mutually conflicting goals with the machine type > > > > definitions. > > > > > > > > Primarily we use them to ensure stable ABI, but an important > > > > secondary goal is to enable new tunables to have new defaults > > > > set, without having to update every mgmt app. The latter > > > > works very well when the defaults have no dependancy on the > > > > platform kernel/OS, but breaks migration when they do have a > > > > platform dependancy. > > > > > > > > > - Firstly, never quietly flipping any bit that affects the ABI... > > > > > > > > > > - Have a default value of off, then QEMU will always allow the VM to boot > > > > > by default, while advanced users can opt-in on new features. We can't > > > > > make this ON by default otherwise some VMs can already fail to boot, > > > > > > > > > > - If the host doesn't support the feature while the cmdline enabled it, > > > > > it needs to fail QEMU boot rather than flipping, so that it says "hey, > > > > > this host does not support running such VM specified, due to XXX > > > > > feature missing". > > > > > > > > > > That's the only way an user could understand what happened, and IMHO that's > > > > > a clean way that we stick with QEMU cmdline on defining the guest ABI, > > > > > while in which the machine type is the fundation of such definition, as the > > > > > machine type can decides many of the rest compat properties. And that's > > > > > the whole point of the compat properties too (to make sure the guest ABI is > > > > > stable). > > > > > > > > > > If kernel breaks it easily, all compat property things that we maintain can > > > > > already stop making sense in general, because it didn't define the whole > > > > > guest ABI.. > > > > > > > > > > So AFAIU that's really what we used for years, I hope I didn't overlook > > > > > somehting. And maybe we don't yet need the "-platform" layer if we can > > > > > keep up with this rule? > > > > > > > > We've failed at this for years wrt enabling use of new defaults that have > > > > a platform depedancy, so historical practice isn't a good reference. > > > > > > > > There are 100's (possibly 1000's) of tunables set implicitly as part of > > > > the machine type, and of those, libvirt likely only exposes a few 10's > > > > of tunables. The vast majority are low level details that no mgmt app > > > > wants to know about, they just want to accept QEMU's new defaults, > > > > while preserving machine ABI. This is a good thing. No one wants the > > > > burden of wiring up every single tunable into libvirt and mgmt apps. > > > > > > > > This is what the "-platform" concept would be intended to preserve. It > > > > would allow a way to enable groups of settings that have a platform level > > > > dependancy, without ever having to teach either libvirt or the mgmt apps > > > > about the individual tunables. > > > > > > Do you think we can achieve similar goal by simply turning the feature to > > > ON only after a few QEMU releases? I also mentioned that idea below. > > > > > > https://lore.kernel.org/r/ZqQNKZ9_OPhDq2AK@x1n > > > > > > So far it really sounds like the right thing to do to me to fix all similar > > > issues, even without introducing anything new we need to maintain. > > > > Turning a feature with a platform dependency to "on" implies that > > the machine type will cease to work out of the box for platforms > > which lack the feature. IMHO that's not acceptable behaviour for > > any of our supported platforms. > > Right, that's why I was thinking whether we should just always be on the > safe side, even if I just replied in the other email to Akihiko, that we do > have the option to make this more aggresive by turning those to ON after > even 1-2 years or even less.. and we have control of how aggressive this > can be. > > > > > IOW, "after a few QEMU releases" implies a delay of as much as > > 5 years, while we wait for platforms which don't support the > > feature to drop out of our supported targets list. I don't > > think that'll satisfy the desire to get the new feature > > available to users as soon as practical for their particular > > platform. > > The feature is always available since the 1st day, right? We just need the > user to opt-in, by specifying ON in the cmdline. > > That'll be my take on this that QEMU's default VM setup should be always > bootable, migratable, and so on. Then user opt-in on stuff like this one, > where there's implication on the ABIs. The "user" can also include > Libvirt. I mean when something is really important, Libvirt should, IMHO, > opt-in by treating that similarly like many cpu properties, and by probing > the host first. > > IIUC there aren't a lot of things like that (part of guest ABI & host > kernel / HW dependent), am I right? Otherwise I would expect more failures > like this one, but it isn't as much as that yet. IIUC it means the efforts > to make Libvirt get involved should be hopefully under control too. The > worst case is Libvirt doesn't auto-on it, but again the user should always > have the option to turn it on when it's necessary. If it is left to libvirt, then it would very likely end up being a user opt-in, not auto-enabled. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|