On Wed, Jan 11, 2023 at 12:40 PM Parav Pandit <pa...@nvidia.com> wrote: > > > > From: Jason Wang <jasow...@redhat.com> > > Sent: Tuesday, January 10, 2023 11:35 PM > > > > On Tue, Jan 10, 2023 at 11:02 AM Parav Pandit <pa...@nvidia.com> wrote: > > > > > > Hi Jason, > > > > > > > From: Jason Wang <jasow...@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.
TX should be fine, since the device can anyhow pretend to send all packets, so we won't have any in-flight descriptors. > > > > > > > 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. Ok, but the production environment should not use that level anyhow. > And regardless, generating zero length packets for debug kernel is even more > confusing. Note that it is allowed in the virtio-spec[1] (we probably can fix that in the driver) and we have pr_debug() all over this drivers and other places. It doesn't cause any side effects except for the debugging purpose. So I think having inflight tracking is useful, but I'm not sure it's worth bothering with virtio-net (or worth to bothering now): - zero length is allowed - it only helps for debugging - may cause issues for migration compatibility - requires new infrastructure to be invented Thanks [1] spec said " Note: len is particularly useful for drivers using untrusted buffers: if a driver does not know exactly how much has been written by the device, the driver would have to zero the buffer in advance to ensure no data leakage occurs. "
