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Subject: Re: [virtio-comment] Re: [PATCH v5] virtio-vsock: add description for datagram type

On Thu, Sep 02, 2021 at 05:08:01PM -0700, Jiang Wang . wrote:
On Thu, Sep 2, 2021 at 7:07 AM Stefan Hajnoczi <stefanha@redhat.com> wrote:

On Thu, Jun 10, 2021 at 06:12:03PM +0000, Jiang Wang wrote:
> Add supports for datagram type for virtio-vsock. Datagram
> sockets are connectionless and unreliable. To avoid contention
> with stream and other sockets, add two more virtqueues and
> a new feature bit to identify if those two new queues exist or not.
> Also add descriptions for resource management of datagram, which
> does not use the existing credit update mechanism associated with
> stream sockets.
> Signed-off-by: Jiang Wang <jiang.wang@bytedance.com>
> ---

Overall this looks good. The tricky thing will be implementing dgram
sockets in a way that minimizes dropped packets and provides some degree
of fairness between senders. Those are implementation issues though and
not visible at the device specification level.

> diff --git a/virtio-vsock.tex b/virtio-vsock.tex
> index da7e641..26a62ac 100644
> --- a/virtio-vsock.tex
> +++ b/virtio-vsock.tex
> @@ -9,14 +9,37 @@ \subsection{Device ID}\label{sec:Device Types / Socket Device / Device ID}
>  \subsection{Virtqueues}\label{sec:Device Types / Socket Device / Virtqueues}
>  \begin{description}
> -\item[0] rx
> -\item[1] tx
> +\item[0] stream rx
> +\item[1] stream tx
> +\item[2] datagram rx
> +\item[3] datagram tx
> +\item[4] event
> +\end{description}
> +The virtio socket device uses 5 queues if feature bit VIRTIO_VSOCK_F_DRGAM is set. Otherwise, it
> +only uses 3 queues, as the following.

s/as the following/as follows:/

Will do.

> +
> +\begin{description}
> +\item[0] stream rx
> +\item[1] stream tx
>  \item[2] event
>  \end{description}
> +When behavior differs between stream and datagram rx/tx virtqueues
> +their full names are used. Common behavior is simply described in
> +terms of rx/tx virtqueues and applies to both stream and datagram
> +virtqueues.
> +
>  \subsection{Feature bits}\label{sec:Device Types / Socket Device / Feature bits}
> -There are currently no feature bits defined for this device.
> +\begin{description}
> +\item[VIRTIO_VSOCK_F_STREAM (0)] Device has support for stream socket type.
> +\end{description}
> +
> +\begin{description}
> +\item[VIRTIO_VSOCK_F_DGRAM (2)] Device has support for datagram socket type.

Is this really bit 2 or did you mean bit 1 (value 0x2)?

I left bit 1 for SEQPACKET feature bit.  That will probably merge
before this patch.

Yep, SEQPACKET implementation is also merged in the linux kernel using the feature bit 1 (0x2), bit 0 (0x1) was left free for stream.

What happens to the virtqueue layout when VIRTIO_VSOCK_F_DGRAM is
present and VIRTIO_VSOCK_F_STREAM is absent? The virtqueue section above
implies that VIRTIO_VSOCK_F_STREAM is always present.

yeah, good question. I  think then it means the first two queues will be used
for dgram?

> +\end{description}
> +
> +If no feature bits are defined, assume device only supports stream socket type.

It's cleaner to define VIRTIO_VSOCK_F_NO_STREAM (0) instead. When the
bit is set the stream socket type is not available and the stream_rx/tx
virtqueues are absent.

This way it's not necessary to define special behavior depending on
certain combinations of feature bits.

Agree. I went back and read old emails again and found I missed the
negative bit part. So repeating the previous question, if
present, then we will only have 3 queues and the first two are for dgram, right?

Also, I am wondering what if an implementation only sets
VIRTIO_VSOCK_F_NO_STREAM bit, but somehow forgot (or for whatever
reason) to set VIRTIO_VSOCK_F_DGRAM bit? Does that mean there will
be no virtqueues? The implementation is a mistake? Because it will not
do anything.
Do we need to explicitly add a sentence in the spec to say something like
"Don't set VIRTIO_VSOCK_F_NO_STREAM alone" etc?

Good point.

IIRC we thought to add F_STREAM to allow devices for example that support only DGRAM, but we said to consider stream supported if no feature was set for backward compatibility.

With F_NO_STREAM we can do the same, but actually we could have this strange case. I don't think it's a big problem, in the end it's a configuration error. Maybe F_NO_STREMA is clearer since the original device spec supports streams without any feature bit defined.


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