RE: [RFC PATCH for 8.0 10/13] virtio-net: Migrate vhost inflight descriptors

[Parav Pandit]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Tuesday, January 10, 2023 11:35 PM
> 
> On Tue, Jan 10, 2023 at 11:02 AM Parav Pandit <parav@nvidia.com> wrote:
> >
> > Hi Jason,
> >
> > > From: Jason Wang <jasowang@redhat.com>
> > > Sent: Monday, December 5, 2022 10:25 PM
> >
> > >
> > > A dumb question, any reason we need bother with virtio-net? It looks
> > > to me it's not a must and would complicate migration compatibility.
> >
> > Virtio net vdpa device is processing the descriptors out of order.
> > This vdpa device doesn’t offer IN_ORDER flag.
> >
> > And when a VQ is suspended it cannot complete these descriptors as some
> dummy zero length completions.
> > The guest VM is flooded with [1].
> 
> Yes, but any reason for the device to do out-of-order for RX?
>
For some devices it is more optimal to process them out of order.
And its not limited to RX.
 
> >
> > So it is needed for the devices that doesn’t offer IN_ORDER feature.
> >
> > [1]
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tre
> > e/drivers/net/virtio_net.c?h=v6.2-rc3#n1252
> 
> It is only enabled in a debug kernel which should be harmless?
it is KERN_DEBUG log level. Its is not debug kernel, just the debug log level.
And regardless, generating zero length packets for debug kernel is even more 
confusing.