Re: [virtio-dev] [PATCH v3] virtio_net: support split header

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Jun 06 2022, Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:

> The purpose of this feature is to split the header and the payload of
> the packet.
>
> |                    receive buffer                                    |
> |                       0th descriptor             | 1th descriptor    |
> | virtnet hdr | mac | ip hdr | tcp hdr|<-- hold -->|   payload         |
>
> We can use a buffer plus a separate page when allocating the receive
> buffer. In this way, we can ensure that all payloads can be
> independently in a page, which is very beneficial for the zerocopy
> implemented by the upper layer.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
>
> 1. Fix some syntax issues
> 2. Fix some terminology issues
> 3. It is not unified with ip alignment, so ip alignment is not included
> 4. Make it clear that the device must support four types, in the case of
> successful negotiation.
>
>  conformance.tex |  2 ++
>  content.tex     | 82 +++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 84 insertions(+)
>

(...)

> diff --git a/content.tex b/content.tex
> index 7508dd1..a5a29d2 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -3084,6 +3084,9 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
>  \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
>      channel.
>
> +\item[VIRTIO_NET_F_SPLIT_HEADER (55)] Device supports to split the protocol
> +    header and the payload.

Maybe s/supports to split/supports splitting/ ?

> +
>  \item[VIRTIO_NET_F_HOST_USO (56)] Device can receive USO packets. Unlike UFO
>   (fragmenting the packet) the USO splits large UDP packet
>   to several segments when each of these smaller packets has UDP header.
> @@ -3131,6 +3134,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
>  \item[VIRTIO_NET_F_CTRL_MAC_ADDR] Requires VIRTIO_NET_F_CTRL_VQ.
>  \item[VIRTIO_NET_F_RSC_EXT] Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_NET_F_HOST_TSO6.
>  \item[VIRTIO_NET_F_RSS] Requires VIRTIO_NET_F_CTRL_VQ.
> +\item[VIRTIO_NET_F_SPLIT_HEADER] Requires VIRTIO_NET_F_CTRL_VQ.
>  \end{description}
>
>  \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Network Device / Feature bits / Legacy Interface: Feature bits}
> @@ -3362,6 +3366,7 @@ \subsection{Device Operation}\label{sec:Device Types / Network Device / Device O
>  #define VIRTIO_NET_HDR_F_NEEDS_CSUM    1
>  #define VIRTIO_NET_HDR_F_DATA_VALID    2
>  #define VIRTIO_NET_HDR_F_RSC_INFO      4
> +#define VIRTIO_NET_HDR_F_SPLIT_HEADER  8
>          u8 flags;
>  #define VIRTIO_NET_HDR_GSO_NONE        0
>  #define VIRTIO_NET_HDR_GSO_TCPV4       1
> @@ -4463,6 +4468,83 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  according to the native endian of the guest rather than
>  (necessarily when not using the legacy interface) little-endian.
>
> +\paragraph{Split Header}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Split Header}
> +
> +If the VIRTIO_NET_F_SPLIT_HEADER feature is negotiated,
> +the device supports to split the protocol header and the payload.

s/supports to split/supports splitting/ ?

> +The protocol header and the payload will be separated into different
> +descriptors.
> +
> +\subparagraph{Split Header}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Split Header / Setting Split Header}
> +
> +To configure the split header, the following layout structure and definitions
> +are used:
> +
> +\begin{lstlisting}
> +struct virtio_net_split_header_config {
> +#define VIRTIO_NET_SPLIT_HEADER_TYPE_TCP4     1
> +#define VIRTIO_NET_SPLIT_HEADER_TYPE_TCP6     2
> +#define VIRTIO_NET_SPLIT_HEADER_TYPE_UDP4     4
> +#define VIRTIO_NET_SPLIT_HEADER_TYPE_UDP6     8

Maybe use the (1 << 0) etc. notation to make it clearer that these are
bits which can be set in different combinations?

> +    le64 type;
> +};
> +
> +#define VIRTIO_NET_CTRL_SPLIT_HEADER       6
> + #define VIRTIO_NET_CTRL_SPLIT_HEADER_SET   0
> +\end{lstlisting}
> +
> +The class VIRTIO_NET_CTRL_SPLIT_HEADER has one command:
> +VIRTIO_NET_CTRL_SPLIT_HEADER_SET applies the new split header configuration.
> +
> +The driver can enable or disable the split header by setting or clearing

Maybe better "The driver can enable or disable split header for
different protocols by..."?

> +corresponding bits in \field{type}.
> +
> +The protocol header contains all headers before level 4 payload. For ip
> +fragmented packets, except for the first fragment, the protocol header only
> +contains all headers before level 3 payload.
> +
> +\devicenormative{\subparagraph}{Setting Split Header}{Device Types / Network Device / Device Operation / Control Virtqueue / Split Header}
> +
> +Split header MUST be disabled upon device reset.

Maybe "A device MUST initialize \field{type} to 0, and MUST set it to 0
upon device reset." ?

> +
> +If VIRTIO_NET_F_SPLIT_HEADER is negotiated, the device MUST support
> +VIRTIO_NET_SPLIT_HEADER_TYPE_TCP4, VIRTIO_NET_SPLIT_HEADER_TYPE_TCP6,
> +VIRTIO_NET_SPLIT_HEADER_TYPE_UDP4, VIRTIO_NET_SPLIT_HEADER_TYPE_UDP6.
> +
> +A device MUST NOT perform split header in the following cases:

Maybe s/perform split header/split the header/ ?

> +\begin{itemize}
> +    \item device does not recognize the protocol of the packet.
> +    \item \field{type} does not include the protocol of the packet.
> +    \item the buffer consists of only one descriptor.
> +    \item the virtio net header and the protocol header exceeds the size
> +        of the first descriptor.

The sum of both, or either of them?

> +    \item If VIRTIO_NET_F_MRG_RXBUF is not negotiated and the size of the

s/If/when/ ?

> +        payload exceeds the size of the buffer chain starting from the 2nd
> +        descriptor.
> +\end{itemize}
> +
> +If the header is split by the device, the \field{flags} of structure
> +virtio_net_hdr MUST contains VIRTIO_NET_HDR_F_SPLIT_HEADER. The protocol header
> +MUST be on the buffer specified by the first descriptor, following the virtio
> +net header. The payload MUST starts from the buffer of the second descriptor.
> +The device MUST set \field{hdr_len} of structure virtio_net_hdr to the length of
> +the protocol header.
> +
> +If the header is split by the device, VIRTIO_NET_F_MRG_RXBUF is negotiated,
> +and the receive packet is spread over multiple buffers, when the device uses a
> +buffer other than the first, if the buffer consists of multiple descriptors, the
> +device MUST skip the buffer of the first descriptor, because the buffer of the
> +first descriptor is used to carry the protocol header.
> +
> +\drivernormative{\subparagraph}{Setting Split Header}{Device Types / Network Device / Device Operation / Control Virtqueue / Split Header}
> +
> +If VIRTIO_NET_HDR_F_SPLIT_HEADER bit in \field{flags} is set, the driver MUST

s/If/If the/

> +trust \field{hdr_len}.

What does "trust" mean? Something like "the driver MUST treat the
contents of \field{hdr_len} as the length of the packet"?

> +
> +If the split header is enabled, the buffers submitted to receiveq by the
> +driver SHOULD at least be composed of two descriptors. The buffer specified by

"SHOULD be composed of at least two descriptors"

> +the first descriptor SHOULD be able to accommodate the virtio net header and the
> +protocol header.
>
>  \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
>  Types / Network Device / Legacy Interface: Framing Requirements}


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