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Subject: Re: [virtio-comment] [RFC v3] virtio-vsock: add description for datagram type
Re-send my thoughts on this new series... On Wed, May 26, 2021 at 05:50:35PM +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.
btw: send the same patch again to include virtio-comment@
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. Thanks, Stefano
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