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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=216.205.24.124; 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=-0.745, 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?B?LiBCZXJyYW5nw6k=?= , 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" On Wed, Aug 25, 2021 at 02:28:55PM +0200, Markus Armbruster wrote: > 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. Hmm... I just found that we don't have such machinery, do we? This does not really work: $ ./qemu-system-x86_64 -M q35 -device virtio-net-pci,bus=pcie.1 \ -device pcie-root-port,id=pcie.1,bus=pcie.0 qemu-system-x86_64: -device virtio-net-pci,bus=pcie.1: Bus 'pcie.1' not found While this will: $ ./qemu-system-x86_64 -M q35 -device pcie-root-port,id=pcie.1,bus=pcie.0 \ -device virtio-net-pci,bus=pcie.1 Thanks, -- Peter Xu