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

[Eduardo Habkost]

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)

How many device types in QEMU have non-default vmsd 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.

-- 
Eduardo