[virtio-dev] Re: [PATCH v2] virtio_net: support split header

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

On Sat, May 07, 2022 at 03:15:33PM +0800, Xuan Zhuo 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>

Okay, but I don't think this covers all possible use-cases.
If we are doing this let's try to address alignment requirements too.

> ---
>  conformance.tex |  2 ++
>  content.tex     | 72 +++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 74 insertions(+)
> 
> diff --git a/conformance.tex b/conformance.tex
> index 663e7c3..6f561fb 100644
> --- a/conformance.tex
> +++ b/conformance.tex
> @@ -149,6 +149,7 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
>  \item \ref{drivernormative:Device Types / Network Device / Device Operation / Control Virtqueue / Automatic receive steering in multiqueue mode}
>  \item \ref{drivernormative:Device Types / Network Device / Device Operation / Control Virtqueue / Offloads State Configuration / Setting Offloads State}
>  \item \ref{drivernormative:Device Types / Network Device / Device Operation / Control Virtqueue / Receive-side scaling (RSS) }
> +\item \ref{drivernormative:Device Types / Network Device / Device Operation / Control Virtqueue / Split Header}
>  \end{itemize}
>  
>  \conformance{\subsection}{Block Driver Conformance}\label{sec:Conformance / Driver Conformance / Block Driver Conformance}
> @@ -411,6 +412,7 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
>  \item \ref{devicenormative:Device Types / Network Device / Device Operation / Control Virtqueue / Gratuitous Packet Sending}
>  \item \ref{devicenormative:Device Types / Network Device / Device Operation / Control Virtqueue / Automatic receive steering in multiqueue mode}
>  \item \ref{devicenormative:Device Types / Network Device / Device Operation / Control Virtqueue / Receive-side scaling (RSS) / RSS processing}
> +\item \ref{devicenormative:Device Types / Network Device / Device Operation / Control Virtqueue / Split Header}
>  \end{itemize}
>  
>  \conformance{\subsection}{Block Device Conformance}\label{sec:Conformance / Device Conformance / Block Device Conformance}
> diff --git a/content.tex b/content.tex
> index 060bdab..3340402 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -3092,6 +3092,9 @@ \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 header and
> +    the payload.
> +
>  \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.
> @@ -3139,6 +3142,7 @@ \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}
> @@ -3370,6 +3374,7 @@ \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
> @@ -4471,6 +4476,73 @@ \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 header and the payload.
> +The header and payload will be separated into different buffers.
> +
> +\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_TCPv4     1
> +#define VIRTIO_NET_SPLIT_HEADER_TYPE_TCPv6     2
> +#define VIRTIO_NET_SPLIT_HEADER_TYPE_UDPv4     4
> +#define VIRTIO_NET_SPLIT_HEADER_TYPE_UDPv6     8
> +    le64 type;
> +};
> +


how do we know what types does device support?

> +#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.
> +
> +\field{type} passed as command data is a bitmask, bits set define
> +packet types to split header, bits cleared - split header to be disabled.
> +
> +The header contains the struct virtio_net_hdr and the header of the package.
> +Such as \field{VIRTIO_NET_SPLIT_HEADER_TYPE_TCPv4} specified header contains
> +virtio_net_hdr, MAC header, IPv4 header (including IPv4 options), TCP header
> +(include TCP options). The back part is the payload.
> +
> +\devicenormative{\subparagraph}{Setting Split Header}{Device Types / Network Device / Device Operation / Control Virtqueue / Split Header}
> +
> +Split header MUST be disabled after device initialization.
> +
> +A device MUST NOT perform split header in the following cases:
> +\begin{itemize}
> +    \item device does not recognize protocol of the packet.
> +    \item \field{type} does not include the protocol of the packet.
> +    \item the packet is a IP fragmentation.
> +    \item the receive buffer consists of only one descriptor.
> +    \item the header exceeds the size of the 0th descriptor.
> +    \item If VIRTIO_NET_F_MRG_RXBUF is not negotiated and the size of the
> +        payload is greater than the total size of the 1th\ldots Nth descriptor.
> +\end{itemize}
> +
> +If the split header completed, then the \field{flags} of virtnet hdr MUST
> +contains VIRTIO_NET_HDR_F_SPLIT_HEADER. The header MUST is on the buffer of the
> +0th descriptor, and the payload MUST starts from the buffer of the 1th descriptor.
> +The device MUST set \field{hdr_len} of virtnet hdr.
> +
> +If VIRTIO_NET_F_MRG_RXBUF is negotiated and the device is to use multiple
> +receive buffers, each subsequent receive buffer MUST skip the 0th descriptor.
> +
> +\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
> +believe \field{hdr_len}, the length of the header in the 0th descriptor is equal
> +to the length of struct virtio_net_hdr plus \field{hdr_len}.
> +
> +If the split header function is enable, the buffers submitted by the driver
> +SHOULD at least be composed of two descriptors. The buffer specified by the 0th
> +descriptor SHOULD be able to accommodate the header.
>

I am not sure I understand what all this is saying. Lots of this is
ungrammatical. Also - I do not understand what do you mean believe,
or 0th descriptor.  Also pls formulate in terms of buffers not descriptors. 

Thanks!
  
>  \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
>  Types / Network Device / Legacy Interface: Framing Requirements}
> -- 
> 2.31.0


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