Re: [RFC v2] virtio-vsock: add description for datagram type

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Apr 13, 2021 at 02:58:53PM +0200, Stefano Garzarella wrote:
> On Mon, Apr 12, 2021 at 03:42:23PM -0700, Jiang Wang . wrote:
> > On Mon, Apr 12, 2021 at 7:21 AM Stefano Garzarella <sgarzare@redhat.com> wrote:
> > > 
> > > On Mon, Apr 12, 2021 at 02:50:17PM +0100, Stefan Hajnoczi wrote:
> > > >On Thu, Apr 01, 2021 at 04:36:02AM +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>
> > > >> ---
> > > >> V2 addressed the comments for the previous version.
> > > >>
> > > >>  virtio-vsock.tex | 62 +++++++++++++++++++++++++++++++++++++++++++++++---------
> > > >>  1 file changed, 52 insertions(+), 10 deletions(-)
> > > >>
> > > >> diff --git a/virtio-vsock.tex b/virtio-vsock.tex
> > > >> index da7e641..62c12e0 100644
> > > >> --- a/virtio-vsock.tex
> > > >> +++ b/virtio-vsock.tex
> > > >> @@ -11,12 +11,25 @@ \subsection{Virtqueues}\label{sec:Device Types / Socket Device / Virtqueues}
> > > >>  \begin{description}
> > > >>  \item[0] rx
> > > >>  \item[1] 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. Rx and tx queues are always used for stream sockets.
> > > >> +
> > > >> +\begin{description}
> > > >> +\item[0] rx
> > > >> +\item[1] tx
> > > >>  \item[2] event
> > > >>  \end{description}
> > > >>
> > > >
> > > >I suggest renaming "rx" and "tx" to "stream rx" and "stream tx"
> > > >virtqueues and also adding this:
> > > >
> > > >  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.
> > > >
> > > >This way you won't need to duplicate portions of the spec that deal with
> > > >populating the virtqueues, for example.
> > > >
> > > >It's also clearer to use a full name for stream rx/tx virtqueues instead
> > > >of calling them rx/tx virtqueues now that we have datagram rx/tx
> > > >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_DGRAM (0)] Device has support for datagram socket type.
> > > >> +\end{description}
> > > >>
> > > >>  \subsection{Device configuration layout}\label{sec:Device Types / Socket Device / Device configuration layout}
> > > >>
> > > >> @@ -107,6 +120,9 @@ \subsection{Device Operation}\label{sec:Device Types / Socket Device / Device Op
> > > >>
> > > >>  \subsubsection{Virtqueue Flow Control}\label{sec:Device Types / Socket Device / Device Operation / Virtqueue Flow Control}
> > > >>
> > > >> +Flow control applies to stream sockets; datagram sockets do not have
> > > >> +flow control.
> > > >> +
> > > >>  The tx virtqueue carries packets initiated by applications and replies to
> > > >>  received packets.  The rx virtqueue carries packets initiated by the device and
> > > >>  replies to previously transmitted packets.
> > > >> @@ -140,12 +156,15 @@ \subsubsection{Addressing}\label{sec:Device Types / Socket Device / Device Opera
> > > >>  consists of a (cid, port number) tuple. The header fields used for this are
> > > >>  \field{src_cid}, \field{src_port}, \field{dst_cid}, and \field{dst_port}.
> > > >>
> > > >> -Currently only stream sockets are supported. \field{type} is 1 for stream
> > > >> -socket types.
> > > >> +Currently stream and datagram (dgram) sockets are supported. \field{type} is 1 for stream
> > > >> +socket types. \field{type} is 3 for dgram socket types.
> > > >>
> > > >>  Stream sockets provide in-order, guaranteed, connection-oriented delivery
> > > >>  without message boundaries.
> > > >>
> > > >> +Datagram sockets provide connectionless unreliable messages of
> > > >> +a fixed maximum length.
> > > >
> > > >Plus unordered (?) and with message boundaries. In other words:
> > > >
> > > >  Datagram sockets provide unordered, unreliable, connectionless message
> > > >  with message boundaries and a fixed maximum length.
> > > >
> > > >I didn't think of the fixed maximum length aspect before. I guess the
> > > >intention is that the rx buffer size is the message size limit? That's
> > > >different from UDP messages, which can be fragmented into multiple IP
> > > >packets and can be larger than 64KiB:
> > > >https://en.wikipedia.org/wiki/User_Datagram_Protocol#UDP_datagram_structure
> > > >
> > > >Is it possible to support large datagram messages in vsock? I'm a little
> > > >concerned that applications that run successfully over UDP will not be
> > > >portable if vsock has this limitation because it would impose extra
> > > >message boundaries that the application protocol might not tolerate.
> > > 
> > > Maybe we can reuse the same approach Arseny is using for SEQPACKET.
> > > Fragment the packets according to the buffers in the virtqueue and set
> > > the EOR flag to indicate the last packet in the message.
> > > 
> > Agree. Another option is to use the ones for skb since we may need to
> > use skbs for multiple transport support anyway.
> > 
> 
> The important thing I think is to have a single flag in virtio-vsock that
> identifies pretty much the same thing: this is the last fragment of a series
> to rebuild a packet.
> 
> We should reuse the same flag for DGRAM and SEQPACKET.
> 
> Thanks,
> Stefano

Well DGRAM can drop data so I wonder whether it can work ...

-- 
MST

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