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

[Stefano Garzarella <sgarzare@redhat.com>]

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
> VIRTIO_VSOCK_F_NO_STREAM  and VIRTIO_VSOCK_F_DGRAM
> 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.

Stefano