Re: [PATCH 2/2] vfio/migration: Parallelize device state transitions

[Peter Xu <peterx@redhat.com>]

On Tue, Jul 14, 2026 at 10:23:05PM +0200, Maciej S. Szmigiero wrote:
> The VFIO devices in {PRE_COPY,RUNNING}_P2P states can {still,already} accept
> DMA so in the master branch by the time prepare_cb() handlers exit and
> cb() ones start running the VFIO devices are {still,already} capable of
> accepting incoming DMA.
> 
> On the other hand, if you meant the state transition into STOP_COPY on the
> migration source currently initiated from cb() handler then there can be an
> ordering issue in principle.
> 
> It looks like the current master branch mostly avoids this by registering
> vhost-net vm_state_change handler from VirtIODevice and not from its parent
> PCI device, so with the usual PCI topology vhost-net cb() handler will run
> before the VFIO PCI device one (and so still hit DMA-capable VFIO device)
> due to the vhost-net VirtIODevice being one level deeper in the qdev tree.
> It could theoretically be problem with more complex PCI topology though.

That's an interesting observation, but neither do I think it's intentional,
nor do I think it guarantees the depth ordering. Consider when a VFIO
device is put under a few layers of PCIe switches, or PCI bridges.  In that
case the VFIO device notifier can have larger depth value v.s. the virtio
device when the virtio device is attached to the root complex.

> 
> > It is also an issue then even before P2P_DMA state introduced, because
> > before that all cb()s can be run at random orders.
> 
> Technically, the order is based on the device qdev tree depth and within
> the same qdev tree depth level it looks like the handler calling order will
> be dependent on the order the registering devices were instantiated.
> 
> This means that even before these P2P states were introduced
> VM configurations that instantiated VFIO/virtio devices in the certain
> "right" order or topology weren't affected.
> 
> So maybe adding these P2P states mostly fixed this issue too?

I still think they're two different issues, and I still think with/without
P2P the issue is the same, now I tend to agree this issue exists,
especially if virtio-blk have similar behavior like virtio-net.

For one example of virtio-blk DMA to VFIO MMIO regions:

https://lore.kernel.org/r/20260616052552.389021-1-gshan@redhat.com

Said that, I don't know it's 100% triggerable in this case with notifiers,
but sounds relevant.  For virtio-net and others, I have less idea.

In all cases, I still think there are two issues to fix, and I think we
don't need to fix this problem in one shot. I still don't think they're
directly relevant, at least on the goals.

So with what I suggested previously relying on prepare_cb() I believe we
can still do the concurrency issue you're looking for, making sure when
reaching VFIO's cb() it's still at least accepting DMAs (P2P is fine).

For the other issue you reported, even if existed, I am not yet sure VFIO
is the only special case that got affected.  I think it's possible some
other emulated devices suffer the same even if it has no direct hardware
attached.  Say, there can be device backends got stopped in cb() when VM
stopped, further DMA to it may cause assertions, then it's the same issue
that needs solving, where essentially we may require all such devices to
provide similar P2P states like VFIO to make sure they initiate DMAs and
flush them in a prepare_cb(), then if all devices' prepare_cb() will make
sure no DMA to be initiated anymore, cb()s will have no ordering constraint
on DMAs.

Thanks,

-- 
Peter Xu