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Mon, 23 Aug 2021 19:51:49 -0700 (PDT) X-Google-Smtp-Source: ABdhPJw1h1JAR65WQB8Niyx/GiQym2hbqCBXvzj2kbsB4+S6X9+DjC4hjKNw9soB2d5dYEPRfUEVvHFRY8mU0lfLhP8= X-Received: by 2002:a2e:2414:: with SMTP id k20mr29866717ljk.482.1629773508786; Mon, 23 Aug 2021 19:51:48 -0700 (PDT) MIME-Version: 1.0 References: <20210818194217.110451-1-peterx@redhat.com> <20210818194318.110993-1-peterx@redhat.com> <20210823184912.mazqfn7gurntj7ld@habkost.net> In-Reply-To: From: Jason Wang Date: Tue, 24 Aug 2021 10:51:37 +0800 Message-ID: Subject: Re: [PATCH 4/4] vl: Prioritize realizations of devices To: Peter Xu Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=jasowang@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Received-SPF: pass client-ip=216.205.24.124; envelope-from=jasowang@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: =?UTF-8?Q?Daniel_P_=2E_Berrang=C3=A9?= , Eduardo Habkost , "Michael S . Tsirkin" , Markus Armbruster , qemu-devel , 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 Tue, Aug 24, 2021 at 3:18 AM 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.. Yes, and I wonder if we limit this to new machine types, we don't even need to care about ABI stuff. Thanks > > > > > 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 | > |--------------------+---------| > > 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. > > From that pov, maybe this patchset would be great if it can be accepted and > applied in early stage of a release? So we can figure out what's missing and > fix them within the same release. However again I still doubt whether there's > any user that will break in a bad way. > > Thanks, > > -- > Peter Xu >