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

[Stefan Hajnoczi <stefanha@redhat.com>]

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:/

> +
> +\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)?

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.

> +\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.

>  \subsubsection{Receive and Transmit}\label{sec:Device Types / Socket Device / Device Operation / Receive and Transmit}
> -The driver queues outgoing packets on the tx virtqueue and incoming packet
> +The driver queues outgoing packets on the tx virtqueue and allocates incoming packet
>  receive buffers on the rx virtqueue. Packets are of the following form:

This change seems unrelated to dgram sockets. I don't think adding the
word "allocates" makes things clearer or more precise. The driver may
reuse receive buffers rather than allocating fresh buffers. I suggest
dropping this change.

>  
>  \begin{lstlisting}
> @@ -195,6 +235,7 @@ \subsubsection{Receive and Transmit}\label{sec:Device Types / Socket Device / De
>  };
>  \end{lstlisting}
>  
> +
>  Virtqueue buffers for outgoing packets are read-only. Virtqueue buffers for
>  incoming packets are write-only.
>  

Unnecessary whitespace change. Please drop.

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