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Mon, 23 Aug 2021 19:52:35 -0700 (PDT) X-Google-Smtp-Source: ABdhPJz0PPitvuKHbVo0QYU3AcG8u5merKAUPKb9NQor1e+49nmg6wDW6YbYdQqwmF8ixThBB7m2LgTdjd8omjbJnDk= X-Received: by 2002:a19:c3d8:: with SMTP id t207mr8116656lff.436.1629773555251; Mon, 23 Aug 2021 19:52:35 -0700 (PDT) MIME-Version: 1.0 References: <20210818194217.110451-1-peterx@redhat.com> <20210818194318.110993-1-peterx@redhat.com> <20210823184912.mazqfn7gurntj7ld@habkost.net> <20210823175457-mutt-send-email-mst@kernel.org> In-Reply-To: From: Jason Wang Date: Tue, 24 Aug 2021 10:52:24 +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" , David Hildenbrand , qemu-devel , Markus Armbruster , 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 6:37 AM Peter Xu wrote: > > 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. It looks to me this doesn't solve the issue of using virtio-mmio with vhost? Thanks > > Thanks, > > -- > Peter Xu >