Linux Virtualization - Apr 2023 - [RFC PATCH net 0/3] virtio-net: allow usage of small vrings

On Sun, Apr 30, 2023 at 04:15:15PM +0300, Alvaro Karsz
wrote:
> 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.
Why the difference?
> 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'd keep current behaviour.
> 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 at
solid-run.com/
> 
> 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

> > 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.
> 
> Why the difference?
> 
Because the RING_SIZE < 4 case requires much more adjustments.

* We may need to squeeze the virtio header into the headroom.
* We may need to squeeze the GSO header into the headroom, or block the
features.
* At the moment, without NETIF_F_SG, we can receive a skb with 2 segments, we
may need to reduce it to 1.
* We may need to change all the control commands, so class,  command and command
specific data will fit in a single segment.
* We may need to disable the control command and all the features depending on
it.
* We may need to disable NAPI?

There may be more changes..

I was thinking that it may be easier to start with the easier case RING_SIZE
>= 4, make sure everything is working fine, then send a follow up patchset
with the required adjustments for RING_SIZE < 4.