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=-3.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 8F7EEC4338F for ; Wed, 25 Aug 2021 12:30:16 +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 24F24610E5 for ; Wed, 25 Aug 2021 12:30:16 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 24F24610E5 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]:43092 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mIs2o-0006ZJ-QZ for qemu-devel@archiver.kernel.org; Wed, 25 Aug 2021 08:30:14 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:43614) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mIs1v-0005tk-6Q for qemu-devel@nongnu.org; Wed, 25 Aug 2021 08:29:19 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:57806) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mIs1q-0006BN-TC for qemu-devel@nongnu.org; Wed, 25 Aug 2021 08:29:17 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1629894552; 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=p5FZGR5Pd59tZZ8fz67/soeSk4/Y1HU3ELxmSk3Wp94=; b=b+bu1bOlb/e2d8OztY3cwKOBUk0f325u4FFpML4e+DDrdA76dltHwQMGGHcfOdJQ6fFfo9 EHM9wUndNKBspkDx2hNlrNVfb22ulU5Cu9k5lIo6zfI35TdMtDLaWINZGvKMnjGsKuVmec w8P9tbPBn3ncE9ssHzUuYo3DVzc7ZsA= 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-139-Hha9HmIlMoqcgR35t1-qYg-1; Wed, 25 Aug 2021 08:29:11 -0400 X-MC-Unique: Hha9HmIlMoqcgR35t1-qYg-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 996C58015C7 for ; Wed, 25 Aug 2021 12:29:10 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-112-4.ams2.redhat.com [10.36.112.4]) by smtp.corp.redhat.com (Postfix) with ESMTPS id A4AD560C82; Wed, 25 Aug 2021 12:28:56 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 3E5DD11380A9; Wed, 25 Aug 2021 14:28:55 +0200 (CEST) From: Markus Armbruster To: Peter Xu Subject: Re: [PATCH 4/4] vl: Prioritize realizations of devices References: <20210818194217.110451-1-peterx@redhat.com> <20210818194318.110993-1-peterx@redhat.com> <20210823184912.mazqfn7gurntj7ld@habkost.net> <20210823210703.cikdkhvyeqqypaqa@habkost.net> <20210823215623.bagyo3oojdpk3byj@habkost.net> <8735qxhnhn.fsf@dusky.pond.sub.org> Date: Wed, 25 Aug 2021 14:28:55 +0200 In-Reply-To: <8735qxhnhn.fsf@dusky.pond.sub.org> (Markus Armbruster's message of "Wed, 25 Aug 2021 11:39:32 +0200") Message-ID: <87h7fdg12w.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=216.205.24.124; envelope-from=armbru@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=-0.745, 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?Q?Berrang=C3=A9?=" , Eduardo Habkost , "Michael S . Tsirkin" , Jason Wang , qemu-devel@nongnu.org, "Dr . David Alan Gilbert" , Eric Auger , Alex Williamson , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Markus Armbruster writes: > Peter Xu writes: > >> On Mon, Aug 23, 2021 at 05:56:23PM -0400, Eduardo Habkost wrote: >>> I don't have any other example, but I assume address assignment >>> based on ordering is a common pattern in device code. >>> >>> I would take a very close and careful look at the devices with >>> non-default vmsd priority. If you can prove that the 13 device >>> types with non-default priority are all order-insensitive, a >>> custom sort function as you describe might be safe. >> >> Besides virtio-mem-pci, there'll also similar devfn issue with all >> MIG_PRI_PCI_BUS, as they'll be allocated just like other pci devices. Say, >> below two cmdlines will generate different pci topology too: >> >> $ qemu-system-x86_64 -device pcie-root-port,chassis=0 \ >> -device pcie-root-port,chassis=1 \ >> -device virtio-net-pci >> >> And: >> >> $ qemu-system-x86_64 -device pcie-root-port,chassis=0 \ >> -device virtio-net-pci >> -device pcie-root-port,chassis=1 \ >> >> This cannot be solved by keeping priority==0 ordering. >> >> After a second thought, I think I was initially wrong on seeing migration >> priority and device realization the same problem. >> >> For example, for live migration we have a requirement on PCI_BUS being migrated >> earlier than MIG_PRI_IOMMU because there's bus number information required >> because IOMMU relies on the bus number to find address spaces. However that's >> definitely not a requirement for device realizations, say, realizing vIOMMU >> after pci buses are fine (bus assigned during bios). >> >> I've probably messed up with the ideas (though they really look alike!). Sorry >> about that. >> >> Since the only ordering constraint so far is IOMMU vs all the rest of devices, >> I'll introduce a new priority mechanism and only make sure vIOMMUs are realized >> earlier. That'll also avoid other implications on pci devfn allocations. >> >> Will rework a new version tomorrow. Thanks a lot for all the comments, > > Is it really a good idea to magically reorder device realization just to > make a non-working command line work? Why can't we just fail the > non-working command line in a way that tells users how to get a working > one? We have way too much ordering magic already... > > If we decide we want more magic, then I'd argue for *dependencies* > instead of priorities. Dependencies are specific and local: $this needs > to go after $that because $reasons. Priorities are unspecific and > global. Having thought about this a bit more... Constraints on realize order are nothing new. For instance, when a device plugs into a bus, it needs to be realized after the device providing the bus. We ensure this by having the device refer to the bus, e.g. bus=pci.0. The reference may be implicit, but it's there. It must resolve for device creation to succeed, and if it resolves, the device providing the bus will be realized in time. I believe what's getting us into trouble with IOMMU is not having such a reference. Or in other words, keeping the dependence between the IOMMU and the devices relying on it *implicit*, and thus hidden from the existing realize-ordering machinery. Instead of inventing another such machinery, let's try to use the one we already have.