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

[Peter Xu]

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 <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..
> > 
> > > 
> > > 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.

Thanks,

-- 
Peter Xu