On Tue, Aug 24, 2021 at 6:37 AM Peter Xu <pet...@redhat.com> 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 <pet...@redhat.com> > > > > > > > > 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 >
