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=-8.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 8940EC4338F for ; Mon, 23 Aug 2021 22:38:07 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 2803061374 for ; Mon, 23 Aug 2021 22:38:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 2803061374 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=nongnu.org Received: from localhost ([::1]:38086 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mIIZy-00021D-DJ for qemu-devel@archiver.kernel.org; Mon, 23 Aug 2021 18:38:06 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:42926) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mIIYx-0000X4-Qq for qemu-devel@nongnu.org; Mon, 23 Aug 2021 18:37:03 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:33049) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mIIYt-0006K3-7A for qemu-devel@nongnu.org; Mon, 23 Aug 2021 18:37:02 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1629758217; 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=QL2dwMT5DHRBybljZ/MyV/EV83bDoCUkhA6FkHKWVps=; b=Fieu/7nX2WnjHH+HVmjWz0SG/hWceHyRG1cnweLeuiJyyrOgNV4046tfoLwPs5OlcouioP uU/+I5cYPd911AugBGX+izHqDXLiBCgxC0zg1wAcVKe9p7v7YSnTl987aBWlBGHv+xAvo8 PBDZasFplx3TcOCfROCuouOfoBnhB50= Received: from mail-qv1-f69.google.com (mail-qv1-f69.google.com [209.85.219.69]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-178-N0YEJG8gMYafgO4sc-SRoQ-1; Mon, 23 Aug 2021 18:36:56 -0400 X-MC-Unique: N0YEJG8gMYafgO4sc-SRoQ-1 Received: by mail-qv1-f69.google.com with SMTP id ib9-20020a0562141c8900b003671c3a1243so7793875qvb.21 for ; Mon, 23 Aug 2021 15:36:56 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=QL2dwMT5DHRBybljZ/MyV/EV83bDoCUkhA6FkHKWVps=; b=jYjuFzj7KFEgQas6Xlv8K8Fq6jN39Hgw4k045n8N/ovY9JIn1eKYLiM5R0e0QeXVle FwoHnGIpZ7jSUru63e3fq3qHZzfEjTT/LxeJO08CQQW4T9qKDpsJm/+DDzroD5rambYR CfZOQasST5aRlWSq1b9daAwV1PP6cR+szEwdiQKmttVGF3BbbhWXOKBw0wQmpbVq6zFk d7ZEWdykxsxzjAxBw7zjEvNqfRyKfwF2C8xuV0ALH8lnsCnQNDhY1ZrG4HXNhCaJfZUP j/bWQWjyyUPbQL3Jp+nO1j0/X5UsN1TVYgiptSPfyKuYJQpIa8bQ29ZgynY9ZlwV8TAM evoQ== X-Gm-Message-State: AOAM533+fyJC3xFOdmBqNZcSTDk3+4pnL4aW7Z7bz0hzEecOfxel3Rmu +lxDH7yV3JQGI2jybgusZTlT26wwY2TwZGg+EC/SKDFCUprI/NjDm/4yqk0e37214R0da8TLx6z lWdKWhUrocEiWfmY= X-Received: by 2002:ac8:682:: with SMTP id f2mr31080561qth.55.1629758216104; Mon, 23 Aug 2021 15:36:56 -0700 (PDT) X-Google-Smtp-Source: ABdhPJxdwiB6O0kCn5ma317eigbh5UNzuxdKD9RQUQhdkUqMHDnKjF7vKb1fdgr4081KnA6j/pB5PQ== X-Received: by 2002:ac8:682:: with SMTP id f2mr31080539qth.55.1629758215888; Mon, 23 Aug 2021 15:36:55 -0700 (PDT) Received: from t490s ([2607:fea8:56a3:500::d413]) by smtp.gmail.com with ESMTPSA id h13sm9257734qkk.67.2021.08.23.15.36.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 23 Aug 2021 15:36:55 -0700 (PDT) Date: Mon, 23 Aug 2021 18:36:53 -0400 From: Peter Xu To: "Michael S. Tsirkin" Subject: Re: [PATCH 4/4] vl: Prioritize realizations of devices Message-ID: References: <20210818194217.110451-1-peterx@redhat.com> <20210818194318.110993-1-peterx@redhat.com> <20210823184912.mazqfn7gurntj7ld@habkost.net> <20210823175457-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 In-Reply-To: <20210823175457-mutt-send-email-mst@kernel.org> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.743, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Daniel P =?utf-8?B?LiBCZXJyYW5nw6k=?= , Eduardo Habkost , David Hildenbrand , Jason Wang , Markus Armbruster , qemu-devel@nongnu.org, Eric Auger , Alex Williamson , Paolo Bonzini , "Dr . David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Aug 23, 2021 at 06:05:07PM -0400, Michael S. Tsirkin wrote: > On Mon, Aug 23, 2021 at 03:18:51PM -0400, Peter Xu wrote: > > On Mon, Aug 23, 2021 at 02:49:12PM -0400, Eduardo Habkost wrote: > > > On Wed, Aug 18, 2021 at 03:43:18PM -0400, Peter Xu wrote: > > > > QEMU creates -device objects in order as specified by the user's cmdline. > > > > However that ordering may not be the ideal order. For example, some platform > > > > devices (vIOMMUs) may want to be created earlier than most of the rest > > > > devices (e.g., vfio-pci, virtio). > > > > > > > > This patch orders the QemuOptsList of '-device's so they'll be sorted first > > > > before kicking off the device realizations. This will allow the device > > > > realization code to be able to use APIs like pci_device_iommu_address_space() > > > > correctly, because those functions rely on the platfrom devices being realized. > > > > > > > > Now we rely on vmsd->priority which is defined as MigrationPriority to provide > > > > the ordering, as either VM init and migration completes will need such an > > > > ordering. In the future we can move that priority information out of vmsd. > > > > > > > > Signed-off-by: Peter Xu > > > > > > Can we be 100% sure that changing the ordering of every single > > > device being created won't affect guest ABI? (I don't think we can) > > > > That's a good question, however I doubt whether there's any real-world guest > > ABI for that. As a developer, I normally specify cmdline parameter in an adhoc > > way, so that I assume most parameters are not sensitive to ordering and I can > > tune the ordering as wish. I'm not sure whether that's common for qemu users, > > I would expect so, but I may have missed something that I'm not aware of. > > > > Per my knowledge the only "guest ABI" change is e.g. when we specify "vfio-pci" > > to be before "intel-iommu": it'll be constantly broken before this patchset, > > while after this series it'll be working. It's just that I don't think those > > "guest ABI" is necessary to be kept, and that's exactly what I want to fix with > > the patchset.. > > > > > > > > How many device types in QEMU have non-default vmsd priority? > > > > Not so much; here's the list of priorities and the devices using it: > > > > |--------------------+---------| > > | priority | devices | > > |--------------------+---------| > > | MIG_PRI_IOMMU | 3 | > > | MIG_PRI_PCI_BUS | 7 | > > | MIG_PRI_VIRTIO_MEM | 1 | > > | MIG_PRI_GICV3_ITS | 1 | > > | MIG_PRI_GICV3 | 1 | > > |--------------------+---------| > > iommu is probably ok. I think virtio mem is ok too, > in that it is normally created by virtio-mem-pci ... Hmm this reminded me whether virtio-mem-pci could have another devfn allocated after being moved.. But frankly I still doubt whether we should guarantee that guest ABI on user not specifying addr=XXX in pci device parameters - I feel like it's a burden that we don't need to carry. (Btw, trying to keep the order is one thing; declare it guest ABI would be another thing to me) > > > > > All the rest devices are using the default (0) priority. > > > > > > > > Can we at least ensure devices with the same priority won't be > > > reordered, just to be safe? (qsort() doesn't guarantee that) > > > > > > If very few device types have non-default vmsd priority and > > > devices with the same priority aren't reordered, the risk of > > > compatibility breakage would be much smaller. > > > > I'm also wondering whether it's a good thing to break some guest ABI due to > > this change, if possible. > > > > Let's imagine something breaks after applied, then the only reason should be > > that qsort() changed the order of some same-priority devices and it's not the > > same as user specified any more. Then, does it also means there's yet another > > ordering requirement that we didn't even notice? > > > > I doubt whether that'll even happen (or I think there'll be report already, as > > in qemu man page there's no requirement on parameter ordering). In all cases, > > instead of "keeping the same priority devices in the same order as the user has > > specified", IMHO we should make the broken devices to have different priorities > > so the ordering will be guaranteed by qemu internal, rather than how user > > specified it. > > Well giving user control of guest ABI is a reasonable thing to do, > it is realize order that users do not really care about. Makes sense. > > I guess we could move pci slot allocation out of realize > so it does not depend on realize order? Yes that sounds like another approach, but it seems to require more changes. Thanks, -- Peter Xu