[RFC PATCH net 0/3] virtio-net: allow usage of small vrings

[Alvaro Karsz]

At the moment, if a virtio network device uses vrings with less than
MAX_SKB_FRAGS + 2 entries, the device won't be functional.

The following condition vq->num_free >= 2 + MAX_SKB_FRAGS will always
evaluate to false, leading to TX timeouts.

This patchset attempts this fix this bug, and to allow small rings down
to 4 entries.

The first patch introduces a new mechanism in virtio core - it allows to
block features in probe time.

If a virtio drivers blocks features and fails probe, virtio core will
reset the device, re-negotiate the features and probe again.

This is needed since some virtio net features are not supported with
small rings.

This patchset follows a discussion in the mailing list [1].

This fixes only part of the bug, rings with less than 4 entries won't
work.
My intention is to split the effort and fix the RING_SIZE < 4 case in a
follow up patchset.

Maybe we should fail probe if RING_SIZE < 4 until the follow up patchset?

I tested the patchset with SNET DPU (drivers/vdpa/solidrun), with packed
and split VQs, with rings down to 4 entries, with and without
VIRTIO_NET_F_MRG_RXBUF, with big MTUs.

I would appreciate more testing.
Xuan: I wasn't able to test XDP with my setup, maybe you can help with
that?

[1] https://lore.kernel.org/lkml/20230416074607.292616-1-alvaro.karsz@xxxxxxxxxxxxx/

Alvaro Karsz (3):
  virtio: re-negotiate features if probe fails and features are blocked
  virtio-net: allow usage of vrings smaller than MAX_SKB_FRAGS + 2
  virtio-net: block ethtool from converting a ring to a small ring

 drivers/net/virtio_net.c | 161 +++++++++++++++++++++++++++++++++++++--
 drivers/virtio/virtio.c  |  73 +++++++++++++-----
 include/linux/virtio.h   |   3 +
 3 files changed, 212 insertions(+), 25 deletions(-)

-- 
2.34.1