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

[Stefano Garzarella <sgarzare@redhat.com>]

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

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