On Wed, Apr 14, 2021 at 08:57:06AM +0200, Stefano Garzarella wrote:
> On Tue, Apr 13, 2021 at 03:58:34PM -0400, Michael S. Tsirkin wrote:
> > On Tue, Apr 13, 2021 at 04:03:51PM +0200, Stefano Garzarella wrote:
> > > On Tue, Apr 13, 2021 at 09:50:45AM -0400, Michael S. Tsirkin wrote:
> > > > On Tue, Apr 13, 2021 at 03:38:52PM +0200, Stefano Garzarella wrote:
> > > > > On Tue, Apr 13, 2021 at 09:16:50AM -0400, Michael S. Tsirkin wrote:
> > > > > > 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
> > > > > > > > <sgarz...@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.w...@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 ...
> > > > > >
> > > > >
> > > > > Yep, this is true, but the channel should not be losing packets, so
> > > > > if the
> > > > > receiver discards packets, it knows that it must then discard all of
> > > > > them
> > > > > until the EOR.
> > > >
> > > > That is not so easy - they can come mixed up from multiple sources.
> > >
> > > I think we can prevent mixing because virtuqueue is point to point and its
> > > use is not thread safe, so the access (in the same peer) is already
> > > serialized.
> > > In the end the packet would be fragmented only before copying it to the
> > > virtuqueue.
> > >
> > > But maybe I missed something...
> >
> > Well I ask what's the point of fragmenting then. I assume it's so we
> > can pass huge messages around so you can't keep locks ...
> >
>
> Maybe I'm wrong, but isn't this similar to what we do in virtio-net with
> mergeable buffers?
The point of mergeable buffers is to use less memory: both for each
packet and for a full receive vq.
> Also in this case I think the fragmentation will happen only in the device,
> since the driver can enqueue the entire buffer.
>
> Maybe we can reuse mergeable buffers for virtio-vsock if the EOR flag is not
> suitable.
That sounds very reasonable.
> IIUC in the vsock device the fragmentation for DGRAM will happen just before
> to queue it in the virtqueue, and the device can check how many buffers are
> available in the queue and it can decide whether to queue them all up or
> throw them away.
> >
> > > > Sure linux net core does this but with fragmentation added in,
> > > > I start wondering whether you are beginning to reinvent the net stack
> > > > ...
> > >
> > > No, I hope not :-), in the end our advantage is that we have a channel
> > > that
> > > doesn't lose packets, so I guess we can make assumptions that the network
> > > stack can't.
> > >
> > > Thanks,
> > > Stefano
> >
> > I still don't know how will credit accounting work for datagram,
> > but proposals I saw seem to actually lose packets ...
> >
>
> I still don't know too, but I think it's not an issue in the RX side,
> since if it doesn't have space, can drop all the fragment.
>
> Another option to avoid fragmentation could be to allocate 64K buffers for
> the new DGRAM virtqueue.
That's a lot of buffers ...
> In this way we will have at most 64K packets, which is similar to UDP/IP,
> without extra work for the fragmentation.
IIRC default MTU is 1280 not 64K ...
>
> Thanks,
> Stefano
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