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 84897C77B7F for ; Wed, 17 May 2023 10:21:30 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pzEHN-0002VE-5W; Wed, 17 May 2023 06:21:09 -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 1pzEHL-0002Tq-Hc for qemu-devel@nongnu.org; Wed, 17 May 2023 06:21:07 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pzEHH-0000uH-P2 for qemu-devel@nongnu.org; Wed, 17 May 2023 06:21:07 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1684318862; 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=RQ/1lf+OSuj2UG0/Hc3Ds1GImqfFV9jIHegJlybS7V0=; b=cMVbw75yjpAIXWDD8vZ2hHNi/yz2ROk140a4r3CR8LoN388HxZnDw96p2kSZ6IE8Wq6L8p KlAEAGtnndJWAb9zXYqLo5HF0X48rt+f67V09nzgT25hCqI9MPNyXx4f7cwlMuuXyLKP6T n4rj4IBCfGSwSPIXMmXIda15G3aMDzU= Received: from mail-wm1-f69.google.com (mail-wm1-f69.google.com [209.85.128.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-376-Cf9XnCfANVCHzl1hXCfHJA-1; Wed, 17 May 2023 06:21:01 -0400 X-MC-Unique: Cf9XnCfANVCHzl1hXCfHJA-1 Received: by mail-wm1-f69.google.com with SMTP id 5b1f17b1804b1-3f42ceb4a4fso2286345e9.0 for ; Wed, 17 May 2023 03:21:01 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1684318860; x=1686910860; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=RQ/1lf+OSuj2UG0/Hc3Ds1GImqfFV9jIHegJlybS7V0=; b=k2AN5Xn7T7dE5kLvtXWm1JbaP9fZj6kWnMZc1TNv7KAD7la1PhatqAq716Y/L5TUm5 cHvZ5oLh8SS5JbKBhqgR11hJyrRdNP58jbVBJ+1oLlcXLdzpaIybLkrX2d5nXBxhJoav VEV0pbko1kJ7rfDmNpYTHEt4DcLElMHkmfAvQk5HlzkOC1m7ie8DaN4YW7nOP5GFEiQy m+6elQUcTykfC8r2cHvKF1xZpgk4rBoaSZX0n97f9LUqMN4Z60kSE694r2URhzjGDOun CFVTONfMWZHc5/OsMs343O5L+MoAe3NazX/xEOGLQwiv/m8wGtaFGB2nh+H5L94MCvbL t4PA== X-Gm-Message-State: AC+VfDzue1HeFKFYcOMVpVLT5UyWpcpAhf5hewkZf3N5yUP7ljpC6YDo yI0UEM5yrI0yVbamobNsxzm/2SkCk/KhOk7RTvDx03cVxNJu8DpffWyP0uQWQXYHvJo93rgjMjh raQ7MIjP3eYjj5Gc= X-Received: by 2002:a5d:554a:0:b0:304:6715:8728 with SMTP id g10-20020a5d554a000000b0030467158728mr280559wrw.18.1684318860032; Wed, 17 May 2023 03:21:00 -0700 (PDT) X-Google-Smtp-Source: ACHHUZ6ErWywMH/zdmzAz2y3gndp9HQo6qEbt07gwdWZAQX7HDc/7AO1biab+3gKakvkcW1c1E4bnA== X-Received: by 2002:a5d:554a:0:b0:304:6715:8728 with SMTP id g10-20020a5d554a000000b0030467158728mr280539wrw.18.1684318859615; Wed, 17 May 2023 03:20:59 -0700 (PDT) Received: from redhat.com ([2.52.6.43]) by smtp.gmail.com with ESMTPSA id k17-20020adff5d1000000b003078681a1e8sm2404304wrp.54.2023.05.17.03.20.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 17 May 2023 03:20:58 -0700 (PDT) Date: Wed, 17 May 2023 06:20:55 -0400 From: "Michael S. Tsirkin" To: Juan Quintela Cc: qemu-devel@nongnu.org, Leonardo Bras , Peter Xu , Jiri Denemark , Avihai Horon , Fiona Ebner , Daniel P =?iso-8859-1?Q?=2E_Berrang=E9?= Subject: Re: [PATCH v3 3/3] migration/doc: We broke backwards compatibility Message-ID: <20230517061843-mutt-send-email-mst@kernel.org> References: <20230515083201.55060-1-quintela@redhat.com> <20230515083201.55060-4-quintela@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230515083201.55060-4-quintela@redhat.com> Received-SPF: pass client-ip=170.10.133.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Mon, May 15, 2023 at 10:32:01AM +0200, Juan Quintela wrote: > When we detect that we have broken backwards compantibility in a > released version, we can't do anything for that version. But once we > fix that bug on the next released version, we can "mitigate" that > problem when migrating to new versions to give a way out of that > machine until it does a hard reboot. > > Signed-off-by: Juan Quintela > --- > docs/devel/migration.rst | 194 +++++++++++++++++++++++++++++++++++++++ > 1 file changed, 194 insertions(+) > > diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst > index 95e797ee60..97b6f48474 100644 > --- a/docs/devel/migration.rst > +++ b/docs/devel/migration.rst > @@ -451,6 +451,200 @@ binary in both sides of the migration. If we use different QEMU > versions process, then we need to have into account all other > differences and the examples become even more complicated. > > +How to mitigate when we have a backward compatibility error > +----------------------------------------------------------- > + > +We broke migration for old machine types continously during > +development. But as soon as we find that there is a problem, we fix > +it. The problem is what happens when we detect after we have done a > +release that something has gone wrong. > + > +Let see how it worked with one example. > + > +After the release of qemu-8.0 we found a problem when doing migration > +of the machine type pc-7.2. > + > +- $ qemu-7.2 -M pc-7.2 -> qemu-7.2 -M pc-7.2 > + > + This migration works > + > +- $ qemu-8.0 -M pc-7.2 -> qemu-8.0 -M pc-7.2 > + > + This migration works > + > +- $ qemu-8.0 -M pc-7.2 -> qemu-7.2 -M pc-7.2 > + > + This migration fails > + > +- $ qemu-7.2 -M pc-7.2 -> qemu-8.0 -M pc-7.2 > + > + This migration fails > + > +So clearly something fails when migration between qemu-7.2 and > +qemu-8.0 with machine type pc-7.2. The error messages, and git bisect > +pointed to this commit. > + > +In qemu-8.0 we got this commit: :: > + > + commit 9a6ef182c03eaa138bae553f0fbb5a123bef9a53 > + Author: Jonathan Cameron > + Date: Thu Mar 2 13:37:03 2023 +0000 > + > + hw/pci/aer: Add missing routing for AER errors > + > +The relevant bits of the commit for our example are this ones: > + > + --- a/hw/pci/pcie_aer.c > + +++ b/hw/pci/pcie_aer.c > + @@ -112,6 +112,10 @@ int pcie_aer_init(PCIDevice *dev, > + > + pci_set_long(dev->w1cmask + offset + PCI_ERR_UNCOR_STATUS, > + PCI_ERR_UNC_SUPPORTED); > + + pci_set_long(dev->config + offset + PCI_ERR_UNCOR_MASK, > + + PCI_ERR_UNC_MASK_DEFAULT); > + + pci_set_long(dev->wmask + offset + PCI_ERR_UNCOR_MASK, > + + PCI_ERR_UNC_SUPPORTED); > + > + pci_set_long(dev->config + offset + PCI_ERR_UNCOR_SEVER, > + PCI_ERR_UNC_SEVERITY_DEFAULT); > + > +The patch changes how we configure pci space for AER. But qemu fails > +when the pci space configuration is different betwwen source and > +destination. > + > +The following commit show how this got fixed: > + > + > + > +The relevant parts of the fix are as follow: > + > +First, we create a new property for the device to be able to configure > +the old behaviour or the new behaviour. :: > + > + diff --git a/hw/pci/pci.c b/hw/pci/pci.c > + index 8a87ccc8b0..5153ad63d6 100644 > + --- a/hw/pci/pci.c > + +++ b/hw/pci/pci.c > + @@ -79,6 +79,8 @@ static Property pci_props[] = { > + DEFINE_PROP_STRING("failover_pair_id", PCIDevice, > + failover_pair_id), > + DEFINE_PROP_UINT32("acpi-index", PCIDevice, acpi_index, 0), > + + DEFINE_PROP_BIT("x-pcie-err-unc-mask", PCIDevice, cap_present, > + + QEMU_PCIE_ERR_UNC_MASK_BITNR, true), > + DEFINE_PROP_END_OF_LIST() > + }; > + > +Notice that we enable te feature for new machine types. > + > +Now we see how the fix is done. This is going to depend on what kind > +of breakage happens, but in this case it is quite simple. :: > + > + diff --git a/hw/pci/pcie_aer.c b/hw/pci/pcie_aer.c > + index 103667c368..374d593ead 100644 > + --- a/hw/pci/pcie_aer.c > + +++ b/hw/pci/pcie_aer.c > + @@ -112,10 +112,13 @@ int pcie_aer_init(PCIDevice *dev, uint8_t cap_ver, > + uint16_t offset, > + > + pci_set_long(dev->w1cmask + offset + PCI_ERR_UNCOR_STATUS, > + PCI_ERR_UNC_SUPPORTED); > + - pci_set_long(dev->config + offset + PCI_ERR_UNCOR_MASK, > + - PCI_ERR_UNC_MASK_DEFAULT); > + - pci_set_long(dev->wmask + offset + PCI_ERR_UNCOR_MASK, > + - PCI_ERR_UNC_SUPPORTED); > + + > + + if (dev->cap_present & QEMU_PCIE_ERR_UNC_MASK) { > + + pci_set_long(dev->config + offset + PCI_ERR_UNCOR_MASK, > + + PCI_ERR_UNC_MASK_DEFAULT); > + + pci_set_long(dev->wmask + offset + PCI_ERR_UNCOR_MASK, > + + PCI_ERR_UNC_SUPPORTED); > + + } > + > + pci_set_long(dev->config + offset + PCI_ERR_UNCOR_SEVER, > + PCI_ERR_UNC_SEVERITY_DEFAULT); > + > +I.e. If the property bit is enabled, we configure it as we did for > +qemu-8.0. If the property bit is not set, we configure it as it was in 7.2. > + > +And now, everything that is missing is disable the feature for old > +machine types: :: > + > + diff --git a/hw/core/machine.c b/hw/core/machine.c > + index 47a34841a5..07f763eb2e 100644 > + --- a/hw/core/machine.c > + +++ b/hw/core/machine.c > + @@ -48,6 +48,7 @@ GlobalProperty hw_compat_7_2[] = { > + { "e1000e", "migrate-timadj", "off" }, > + { "virtio-mem", "x-early-migration", "false" }, > + { "migration", "x-preempt-pre-7-2", "true" }, > + + { TYPE_PCI_DEVICE, "x-pcie-err-unc-mask", "off" }, > + }; > + const size_t hw_compat_7_2_len = G_N_ELEMENTS(hw_compat_7_2); > + > +And now, when qemu-8.0.1 is released with this fix, all combinations > +are going to work as supposed. > + > +- $ qemu-7.2 -M pc-7.2 -> qemu-7.2 -M pc-7.2 (works) > +- $ qemu-8.0.1 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 (works) > +- $ qemu-8.0.1 -M pc-7.2 -> qemu-7.2 -M pc-7.2 (works) > +- $ qemu-7.2 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 (works) > + > +So the normality has been restaured and everything is ok, no? > + > +Not really, now our matrix is much bigger. We started with the easy > +cases, migration from the same version to the same version always > +works: > + > +- $ qemu-7.2 -M pc-7.2 -> qemu-7.2 -M pc-7.2 > +- $ qemu-8.0 -M pc-7.2 -> qemu-8.0 -M pc-7.2 > +- $ qemu-8.0.1 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 > + > +Now the interesting ones. When the QEMU processes versions are > +different. For the 1st set, their fail and we can do nothing, both > +versions are relased and we can't change anything. > + > +- $ qemu-7.2 -M pc-7.2 -> qemu-8.0 -M pc-7.2 > +- $ qemu-8.0 -M pc-7.2 -> qemu-7.2 -M pc-7.2 > + > +This two are the ones that work. The whole point of making the > +change in qemu-8.0.1 release was to fix this issue: > + > +- $ qemu-7.2 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 > +- $ qemu-8.0.1 -M pc-7.2 -> qemu-7.2 -M pc-7.2 > + > +But now we found that qemu-8.0 neither can migrate to qemu-7.2 not > +qemu-8.0.1. > + > +- $ qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 > +- $ qemu-8.0.1 -M pc-7.2 -> qemu-8.0 -M pc-7.2 > + > +So, if we start a pc-7.2 machine in qemu-8.0 we can't migrate it to > +anything except to qemu-8.0. > + > +Can we do better? > + > +Yeap. If we know that we are gonig to do this migration: > + > +- $ qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 > + > +We can launche the appropiate devices with > + > +--device...,x-pci-e-err-unc-mask=on > + > +And now we can receive a migration from 8.0. And from now on, we can > +do that migration to new machine types if we remember to enable that > +property for pc-7.2. Notice that we need to remember, it is not > +enough to know that the source of the migration is qemu-8.0. Think of this example: > + > +$ qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 -> qemu-8.2 -M pc-7.2 > + > +In the second migration, the source is not qemu-8.0, but we still have > +that "problem" and have that property enabled. Notice that we need to > +continue having this mark/property until we have this machine > +rebooted. But it is not a normal reboot (that don't reload qemu) we > +need the mapchine to poweroff/poweron on a fixed qemu. And from now > +on we can use the proper real machine. > + > VMState > ------- Can we release this list of things that need to be configured somewhere? Maybe in a sane format that libvirt can parse? > -- > 2.40.1