Re: [PATCH] virtio-vsock: add VIRTIO_VSOCK_F_DGRAM feature bit

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, Aug 29, 2023 at 09:29:45PM +0000, Bobby Eshleman wrote:
> This adds support for datagrams to the virtio-vsock device.
>
> virtio-vsock already supports stream and seqpacket types. The existing
> message types and header fields are extended to support datagrams.
> Semantic differences between the flow types are stated, as well as any
> additional requirements for devices and drivers implementing this
> feature.
>
> Signed-off-by: Bobby Eshleman <bobby.eshleman@bytedance.com>
> ---
> device-types/vsock/description.tex | 95 +++++++++++++++++++++++++++---
> 1 file changed, 88 insertions(+), 7 deletions(-)
>
> diff --git a/device-types/vsock/description.tex b/device-types/vsock/description.tex
> index 7d91d159872f..638dca8e5da1 100644
> --- a/device-types/vsock/description.tex
> +++ b/device-types/vsock/description.tex
> @@ -20,6 +20,7 @@ \subsection{Feature bits}\label{sec:Device Types / Socket Device / Feature bits}
> \item[VIRTIO_VSOCK_F_STREAM (0)] stream socket type is supported.
> \item[VIRTIO_VSOCK_F_SEQPACKET (1)] seqpacket socket type is supported.
> \item[VIRTIO_VSOCK_F_NO_IMPLIED_STREAM (2)] stream socket type is not implied.
> +\item[VIRTIO_VSOCK_F_DGRAM (3)] datagram socket type is supported.
> \end{description}
>
> \drivernormative{\subsubsection}{Feature bits}{Device Types / Socket Device / Feature bits}
> @@ -167,17 +168,22 @@ \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 stream and seqpacket sockets are supported. \field{type} is 1 (VIRTIO_VSOCK_TYPE_STREAM)
> -for stream socket types, and 2 (VIRTIO_VSOCK_TYPE_SEQPACKET) for seqpacket socket types.
> +
> +Currently stream, seqpacket, and datagram sockets are supported. \field{type} is
> +1 (VIRTIO_VSOCK_TYPE_STREAM) for stream socket types, 2 (VIRTIO_VSOCK_TYPE_SEQPACKET) for
> +seqpacket socket types, and 3 (VIRTIO_VSOCK_TYPE_DGRAM) for datagram socket types.
>
> \begin{lstlisting}
> #define VIRTIO_VSOCK_TYPE_STREAM    1
> #define VIRTIO_VSOCK_TYPE_SEQPACKET 2
> +#define VIRTIO_VSOCK_TYPE_DGRAM     3
> \end{lstlisting}
>
> Stream sockets provide in-order, guaranteed, connection-oriented delivery
> without message boundaries. Seqpacket sockets provide in-order, guaranteed,
> -connection-oriented delivery with message and record boundaries.
> +connection-oriented delivery with message and record boundaries. Datagram
> +sockets provide connection-less, best-effort delivery of messages, with no
> +order or reliability guarantees.
>
> \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
> @@ -203,16 +209,19 @@ \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.
>
> +\field{buf_alloc} and \field{fwd_cnt} are reserved for future use by datagram
> +sockets. These fields are not used for datagram buffer space management.
> +
> \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 and seqpacket flows, VIRTIO_VSOCK_OP_RW data packets MUST only be
> +transmitted when the peer has sufficient free buffer space for the payload.
>
> 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 and seqpacket flows, VIRTIO_VSOCK_OP_RW data packets MUST only be
> +transmitted when the peer has sufficient free buffer space for the payload.
>
> All packets associated with a stream flow MUST contain valid information in
> \field{buf_alloc} and \field{fwd_cnt} fields.
> @@ -299,6 +308,78 @@ \subsubsection{Seqpacket Sockets}\label{sec:Device Types / Socket Device / Devic
> #define VIRTIO_VSOCK_SEQ_EOR (1 << 1)
> \end{lstlisting}
>
> +\subsubsection{Datagram Sockets}\label{sec:Device Types / Socket Device / Device Operation / Datagram Sockets}
> +
> +\drivernormative{\paragraph}{Device Operation: Packet Fragmentation}{Device Types / Socket Device / Datagram Sockets / Fragmentation}
> +
> +Drivers MAY disassemble packets into smaller fragments. If drivers fragment a
> +packet, they MUST follow the fragmentation rules described in section
> +\ref{sec:Device Types / Socket Device / Device Operation / Datagram Sockets / Fragmentation}.
> +
> +Drivers MUST support assembly of received packet fragments according to the
> +fragmentation rules described in section
> +\ref{sec:Device Types / Socket Device / Device Operation / Datagram 
> Sockets / Fragmentation}.
> +
> +\devicenormative{\paragraph}{Device Operation: Packet Fragmentation}{Device Types / Socket Device / Datagram Sockets / Fragmentation}
> +
> +Devices MAY disassemble packets into smaller fragments. If devices fragment a
> +packet, they MUST follow the fragmentation rules described in section
> +\ref{sec:Device Types / Socket Device / Device Operation / Datagram Sockets / Fragmentation}.
> +
> +Devices MUST support assembly of received packet fragments according to the
> +fragmentation rules described in section
> +\ref{sec:Device Types / Socket Device / Device Operation / Datagram Sockets / Fragmentation}.
> +
> +\drivernormative{\paragraph}{Device Operation: Packet Dropping}{Device Types / Socket Device / Datagram Sockets / Dropping}
> +
> +The driver MAY drop received packets with no notification to the device. This
> +can happen if, for example, there are insufficient resources or no socket
> +exists for the destination address.
> +
> +\devicenormative{\paragraph}{Device Operation: Packet Dropping}{Device Types / Socket Device / Datagram Sockets / Dropping}
> +
> +The device MAY drop received packets with no notification to the driver. This
> +can happen if, for example, there are insufficient resources or no socket
> +exists for the destination address.

Should we provide some notification if the socket does not exist at the
destination?

> +
> +\paragraph{Datagram Fragmentation}\label{sec:Device Types / Socket Device / Device Operation / Datagram Sockets / Fragmentation}
> +
> +\field{flags} may have the following bit set:
> +
> +\begin{lstlisting}
> +#define VIRTIO_VSOCK_DGRAM_EOM (1 << 0)
> +\end{lstlisting}
> +
> +When the header \field{flags} field bit VIRTIO_VSOCK_DGRAM_EOM (bit 0) is set,
> +it indicates that the current payload is the end of a datagram 
> fragment OR that
> +the current payload is an entire datagram packet.

In the destination, if we discard some fragments, then could we
reconstruct a different datagram from the one sent?

Is that anything acceptable?

Thanks,
Stefano

> +
> +Datagram fragmentation is subject to the following rules:
> +
> +The fragments for a datagram packet MUST be added to the virtqueue in
> +sequential order.
> +
> +If a packet is not a fragment, then the VIRTIO_VSOCK_DGRAM_EOM bit (bit 0) of
> +\field{flags} MUST be set.
> +
> +If a packet is the last fragment of a fragment sequence, then the
> +VIRTIO_VSOCK_DGRAM_EOM bit (bit 0) of \field{flags} MUST be set.
> +
> +If a packet is any fragment except the last fragment of a fragment sequence,
> +then the VIRTIO_VSOCK_DGRAM_EOM bit (bit 0) of \field{flags} MUST NOT be set.
> +
> +If a driver or device has already placed a fragment of a packet on the
> +virtqueue, it MUST add the remaining fragments of the packet to the virtqueue
> +before adding any additional packets or fragments to the virtqueue.
> +
> +If fragments for a packet are only partially received after an
> +implementation-specific amount of time, then the destination device or driver
> +MAY drop the fragments.
> +
> +Each buffer containing a fragment MUST begin with a valid struct
> +virtio_vsock_hdr. \field{len} MUST equal the length of the fragment payload
> +only.
> +
> \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
> --
> 2.20.1