Subject: Re: [PATCH 0/6] virtio/vsock: introduce dgrams, sk_buff, and qdisc

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 17 Aug 2022 02:54:33 -0400

On Mon, Aug 15, 2022 at 10:56:03AM -0700, Bobby Eshleman wrote:
> Hey everybody,
> 
> This series introduces datagrams, packet scheduling, and sk_buff usage
> to virtio vsock.
> 
> The usage of struct sk_buff benefits users by a) preparing vsock to use
> other related systems that require sk_buff, such as sockmap and qdisc,
> b) supporting basic congestion control via sock_alloc_send_skb, and c)
> reducing copying when delivering packets to TAP.
> 
> The socket layer no longer forces errors to be -ENOMEM, as typically
> userspace expects -EAGAIN when the sk_sndbuf threshold is reached and
> messages are being sent with option MSG_DONTWAIT.
> 
> The datagram work is based off previous patches by Jiang Wang[1].
> 
> The introduction of datagrams creates a transport layer fairness issue
> where datagrams may freely starve streams of queue access. This happens
> because, unlike streams, datagrams lack the transactions necessary for
> calculating credits and throttling.
> 
> Previous proposals introduce changes to the spec to add an additional
> virtqueue pair for datagrams[1]. Although this solution works, using
> Linux's qdisc for packet scheduling leverages already existing systems,
> avoids the need to change the virtio specification, and gives additional
> capabilities. The usage of SFQ or fq_codel, for example, may solve the
> transport layer starvation problem. It is easy to imagine other use
> cases as well. For example, services of varying importance may be
> assigned different priorities, and qdisc will apply appropriate
> priority-based scheduling. By default, the system default pfifo qdisc is
> used. The qdisc may be bypassed and legacy queuing is resumed by simply
> setting the virtio-vsock%d network device to state DOWN. This technique
> still allows vsock to work with zero-configuration.

The basic question to answer then is this: with a net device qdisc
etc in the picture, how is this different from virtio net then?
Why do you still want to use vsock?

> In summary, this series introduces these major changes to vsock:
> 
> - virtio vsock supports datagrams
> - virtio vsock uses struct sk_buff instead of virtio_vsock_pkt
>   - Because virtio vsock uses sk_buff, it also uses sock_alloc_send_skb,
>     which applies the throttling threshold sk_sndbuf.
> - The vsock socket layer supports returning errors other than -ENOMEM.
>   - This is used to return -EAGAIN when the sk_sndbuf threshold is
>     reached.
> - virtio vsock uses a net_device, through which qdisc may be used.
>  - qdisc allows scheduling policies to be applied to vsock flows.
>   - Some qdiscs, like SFQ, may allow vsock to avoid transport layer congestion. That is,
>     it may avoid datagrams from flooding out stream flows. The benefit
>     to this is that additional virtqueues are not needed for datagrams.
>   - The net_device and qdisc is bypassed by simply setting the
>     net_device state to DOWN.
> 
> [1]: https://lore.kernel.org/all/[email-masked]/
> 
> Bobby Eshleman (5):
>   vsock: replace virtio_vsock_pkt with sk_buff
>   vsock: return errors other than -ENOMEM to socket
>   vsock: add netdev to vhost/virtio vsock
>   virtio/vsock: add VIRTIO_VSOCK_F_DGRAM feature bit
>   virtio/vsock: add support for dgram
> 
> Jiang Wang (1):
>   vsock_test: add tests for vsock dgram
> 
>  drivers/vhost/vsock.c                   | 238 ++++----
>  include/linux/virtio_vsock.h            |  73 ++-
>  include/net/af_vsock.h                  |   2 +
>  include/uapi/linux/virtio_vsock.h       |   2 +
>  net/vmw_vsock/af_vsock.c                |  30 +-
>  net/vmw_vsock/hyperv_transport.c        |   2 +-
>  net/vmw_vsock/virtio_transport.c        | 237 +++++---
>  net/vmw_vsock/virtio_transport_common.c | 771 ++++++++++++++++--------
>  net/vmw_vsock/vmci_transport.c          |   9 +-
>  net/vmw_vsock/vsock_loopback.c          |  51 +-
>  tools/testing/vsock/util.c              | 105 ++++
>  tools/testing/vsock/util.h              |   4 +
>  tools/testing/vsock/vsock_test.c        | 195 ++++++
>  13 files changed, 1176 insertions(+), 543 deletions(-)
> 
> -- 
> 2.35.1