Re: [RFC v3] virtio-vsock: add description for datagram type

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

Hmm that's insufficient unfortunately.  According to our bylaws we need
the original contributor to send it to
virtio-comment@lists.oasis-open.org

Jiang can you do it pls? Pls note it's a subscriber-only list.

Thanks!


On Wed, May 26, 2021 at 11:00:57AM +0200, Stefano Garzarella wrote:
> Hi Jiang,
> I think we should discuss this changes on
> virtio-comment@lists.oasis-open.org, so I CCed the list.
> 
> 
> On Sat, May 22, 2021 at 11:44:40PM +0000, Jiang Wang wrote:
> > From: "jiang.wang" <jiang.wang@bytedance.com>
> > 
> > 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.
> > V3: add description for the mergeable receive buffer.
> > 
> > virtio-vsock.tex | 137 +++++++++++++++++++++++++++++++++++++++++++++++++------
> > 1 file changed, 124 insertions(+), 13 deletions(-)
> > 
> > diff --git a/virtio-vsock.tex b/virtio-vsock.tex
> > index da7e641..7eb3596 100644
> > --- a/virtio-vsock.tex
> > +++ b/virtio-vsock.tex
> > @@ -9,14 +9,36 @@ \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.
> > +
> > +\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_DGRAM (0)] Device has support for datagram socket type.
> > +\end{description}
> 
> As suggested by Michael here [1] we should add also a feature bit for
> stream, and maybe is better to reserve bit 0 for it.
> 
> Arseny already sent an implementation of SEQPACKET using bit 1 for
> VIRTIO_VSOCK_F_SEQPACKET, so I think we can use bit 2 for DGRAM and bit 3
> for MRG_RXBUF.
> 
> [1]
> https://lists.oasis-open.org/archives/virtio-comment/202104/msg00016.html
> 
> > +
> > +\begin{description}
> > +\item[VIRTIO_VSOCK_F_MRG_RXBUF (1)] Driver can merge receive buffers.
> > +\end{description}
> > +
> > 
> > \subsection{Device configuration layout}\label{sec:Device Types / Socket Device / Device configuration layout}
> > 
> > @@ -64,6 +86,8 @@ \subsection{Device Operation}\label{sec:Device Types / Socket Device / Device Op
> > 
> > Packets transmitted or received contain a header before the payload:
> > 
> > +If feature VIRTIO_VSOCK_F_MRG_RXBUF is not negotiated, use the following header.
> > +
> > \begin{lstlisting}
> > struct virtio_vsock_hdr {
> > 	le64 src_cid;
> > @@ -79,6 +103,15 @@ \subsection{Device Operation}\label{sec:Device Types / Socket Device / Device Op
> > };
> > \end{lstlisting}
> > 
> > +If feature VIRTIO_VSOCK_F_MRG_RXBUF is negotianted, use the following header.
> > +\begin{lstlisting}
> > +struct virtio_vsock_hdr_mrg_rxbuf {
> > +	struct virtio_vsock_hdr hdr;
> > +	le16 num_buffers;
> > +};
> > +\end{lstlisting}
> > +
> > +
> > The upper 32 bits of src_cid and dst_cid are reserved and zeroed.
> > 
> > Most packets simply transfer data but control packets are also used for
> > @@ -107,6 +140,9 @@ \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 +176,15 @@ \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 unordered, unreliable, connectionless messages
> > +with message boundaries and a maximum length.
> > +
> > \subsubsection{Buffer Space Management}\label{sec:Device Types / Socket Device / Device Operation / Buffer Space Management}
> > \field{buf_alloc} and \field{fwd_cnt} are used for buffer space management of
> > stream sockets. The guest and the device publish how much buffer space is
> > @@ -162,7 +201,7 @@ \subsubsection{Buffer Space Management}\label{sec:Device Types / Socket Device /
> > u32 peer_free = peer_buf_alloc - (tx_cnt - peer_fwd_cnt);
> > \end{lstlisting}
> > 
> > -If there is insufficient buffer space, the sender waits until virtqueue
> > buffers
> > +For stream sockets, if there is insufficient buffer space, the sender waits until virtqueue buffers
> > are returned and checks \field{buf_alloc} and \field{fwd_cnt} again. Sending
> > the VIRTIO_VSOCK_OP_CREDIT_REQUEST packet queries how much buffer space is
> > available. The reply to this query is a VIRTIO_VSOCK_OP_CREDIT_UPDATE
> > packet.
> > @@ -170,22 +209,52 @@ \subsubsection{Buffer Space Management}\label{sec:Device Types / Socket Device /
> > previously receiving a VIRTIO_VSOCK_OP_CREDIT_REQUEST packet. This allows
> > communicating updates any time a change in buffer space occurs.
> > 
> > +Unlike stream sockets, dgram sockets do not use VIRTIO_VSOCK_OP_CREDIT_UPDATE or
> > +VIRTIO_VSOCK_OP_CREDIT_REQUEST packets. The dgram buffer management
> > +is split to two parts: tx side and rx side. For the tx side, if the
> > +virtqueue is full, the packet will be dropped.
> > +For the rx side, dgram also uses the \field{buf_alloc}. If it is full, the packet
> > +is dropped by the receiver.
> > +
> > +\drivernormative{\paragraph}{Device Operation: Buffer Space Management}{Device Types / Socket Device / Device Operation / Setting Up Receive Buffers}
> > +\begin{itemize}
> > +\item If VIRTIO_VSOCK_F_MRG_RXBUF is not negotiated, the driver SHOULD populate the receive queue(s)
> > +      with buffers of at least 1526 bytes for stream sockets and 4096 bytes for datagram sockets.
> > +\item If VIRTIO_VSOCK_F_MRG_RXBUF is negotiated, each buffer MUST be at
> > +least the size of the struct virtio_vsock_hdr.
>                                ^
>                                Should it be virtio_vsock_hdr_mrg_rxbuf?
> 
> > +\end{itemize}
> > +
> > +\begin{note}
> > +Obviously each buffer can be split across multiple descriptor elements.
> > +\end{note}
> > +
> > +\devicenormative{\paragraph}{Device Operation: Buffer Space Management}{Device Types / Socket Device / Device Operation / Setting Up Receive Buffers}
> > +The device MUST set \field{num_buffers} to the number of descriptors used when
> > +transmitting the  packet.
> > +
> > +The device MUST use only a single descriptor if
> > VIRTIO_VSOCK_F_MRG_RXBUF
> > +is not negotiated.
> > +
> > \drivernormative{\paragraph}{Device Operation: Buffer Space Management}{Device Types / Socket Device / Device Operation / Buffer Space Management}
> > -VIRTIO_VSOCK_OP_RW data packets MUST only be transmitted when the peer has
> > -sufficient free buffer space for the payload.
> > +For stream sockets, VIRTIO_VSOCK_OP_RW data packets MUST only be transmitted when the peer has
> > +sufficient free buffer space for the payload. For dgram sockets, VIRTIO_VSOCK_OP_RW data packets
> > +MAY be transmitted when the peer rx buffer is full. Then the packet will be dropped by the peer,
> > +and driver will not get any notification.
> > 
> > All packets associated with a stream flow MUST contain valid information in
> > \field{buf_alloc} and \field{fwd_cnt} fields.
> > 
> > \devicenormative{\paragraph}{Device Operation: Buffer Space Management}{Device Types / Socket Device / Device Operation / Buffer Space Management}
> > -VIRTIO_VSOCK_OP_RW data packets MUST only be transmitted when the peer has
> > -sufficient free buffer space for the payload.
> > +For stream sockets, VIRTIO_VSOCK_OP_RW data packets MUST only be transmitted when the peer has
> > +sufficient free buffer space for the payload. For dgram sockets, VIRTIO_VSOCK_OP_RW data packets
> > +MAY be transmitted when the peer rx buffer is full. Then the packet
> > will be dropped by the peer,
> > +and the device will not get any notification.
> > 
> > All packets associated with a stream flow MUST contain valid information
> > in
> > \field{buf_alloc} and \field{fwd_cnt} fields.
> > 
> > \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:
> > 
> > \begin{lstlisting}
> > @@ -198,21 +267,55 @@ \subsubsection{Receive and
> > Transmit}\label{sec:Device Types / Socket Device / De
> > Virtqueue buffers for outgoing packets are read-only. Virtqueue buffers
> > for
> > incoming packets are write-only.
> > 
> > +When transmitting packets to the device, \field{num_buffers} is not used.
> 
> Should we add a new `struct virtio_vsock_packet` description used when
> VIRTIO_VSOCK_F_MRG_RXBUF is negotiated?
> 
> > +
> > +\begin{enumerate}
> > +\item \field{num_buffers} indicates how many descriptors
> > +  this packet is spread over (including this one): this will
> > +  always be 1 if VIRTIO_VSOCK_F_MRG_RXBUF was not negotiated.
> 
> If VIRTIO_VSOCK_F_MRG_RXBUF was not negotiated, we use only virtio_vsock_hdr
> where there isn't `num_buffers`, so it is not clear to me this statement.
> 
> > +  This allows receipt of large packets without having to allocate large
> > +  buffers: a packet that does not fit in a single buffer can flow
> > +  over to the next buffer, and so on. In this case, there will be
> > +  at least \field{num_buffers} used buffers in the virtqueue, and the device
> > +  chains them together to form a single packet in a way similar to
> > +  how it would store it in a single buffer spread over multiple
> > +  descriptors.
> > +  The other buffers will not begin with a struct virtio_vsock_hdr.
> > +
> > +\item If
> > +  \field{num_buffers} is one, then the entire packet will be
> > +  contained within this buffer, immediately following the struct
> > +  virtio_vsock_hdr.
> > +\end{enumerate}
> > +
> > \drivernormative{\paragraph}{Device Operation: Receive and Transmit}{Device Types / Socket Device / Device Operation / Receive and Transmit}
> > 
> > The \field{guest_cid} configuration field MUST be used as the source CID when
> > sending outgoing packets.
> > 
> > -A VIRTIO_VSOCK_OP_RST reply MUST be sent if a packet is received with an
> > +For stream and datagram sockets, A VIRTIO_VSOCK_OP_RST reply MUST be sent if a packet is received with an
> > unknown \field{type} value.
> > 
> > \devicenormative{\paragraph}{Device Operation: Receive and Transmit}{Device Types / Socket Device / Device Operation / Receive and Transmit}
> > 
> > The \field{guest_cid} configuration field MUST NOT contain a reserved CID as listed in \ref{sec:Device Types / Socket Device / Device configuration layout}.
> > 
> > -A VIRTIO_VSOCK_OP_RST reply MUST be sent if a packet is received with an
> > +For stream and datagram sockets, A VIRTIO_VSOCK_OP_RST reply MUST be sent if a packet is received with an
> > unknown \field{type} value.
> > 
> > +If VIRTIO_VSOCK_F_MRG_RXBUF has been negotiated, the device MUST set
> > +\field{num_buffers} to indicate the number of buffers
> > +the packet (including the header) is spread over.
> > +
> > +If a receive packet is spread over multiple buffers, the device
> > +MUST use all buffers but the last (i.e. the first $\field{num_buffers} -
> > +1$ buffers) completely up to the full length of each buffer
> > +supplied by the driver.
> > +
> > +The device MUST use all buffers used by a single receive
> > +packet together, such that at least \field{num_buffers} are
> > +observed by driver as used.
> > +
> > \subsubsection{Stream Sockets}\label{sec:Device Types / Socket Device / Device Operation / Stream Sockets}
> > 
> > Connections are established by sending a VIRTIO_VSOCK_OP_REQUEST packet. If a
> > @@ -240,6 +343,14 @@ \subsubsection{Stream Sockets}\label{sec:Device Types / Socket Device / Device O
> > destination) address tuple for a new connection while the other peer is still
> > processing the old connection.
> > 
> > +\subsubsection{Datagram Sockets}\label{sec:Device Types / Socket Device / Device Operation / Datagram Sockets}
> > +
> > +Datagram (dgram) sockets are connectionless and unreliable. The sender
> > just sends
> > +a message to the peer and hopes it will be delivered. A VIRTIO_VSOCK_OP_RST reply is sent if
> > +a receiving socket does not exist on the destination.
> > +If the transmission or receiving buffers are full, the packets
> > +are dropped.
> > +
> > \subsubsection{Device Events}\label{sec:Device Types / Socket Device / Device Operation / Device Events}
> > 
> > Certain events are communicated by the device to the driver using the event
> > -- 
> > 2.11.0
> > 
> 
> Thanks,
> Stefano