Re: [PATCH 4/4] vl: Prioritize realizations of devices

[Jason Wang <jasowang@redhat.com>]

On Tue, Aug 24, 2021 at 3:18 AM Peter Xu <peterx@redhat.com> 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 <peterx@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..

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
>