Re: [virtio-dev] [PATCH v5] virtio_net: support split header

[Cornelia Huck <cohuck@redhat.com>]

On Wed, Jul 20 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>
> Reviewed-by: Heng Qi <hengqi@linux.alibaba.com>
> Reviewed-by: Kangjie Xu <kangjie.xu@linux.alibaba.com>
> ---
> v5:
>     1. Determine when hdr_len is credible in the process of rx
>     2. Clean up the use of buffers and descriptors
>     3. Clarify the meaning of used lenght if the first descriptor is skipped in the case of merge
>
> v4:
>     1. fix typo @Cornelia Huck @Jason Wang
>     2. do not split header for IP fragmentation packet. @Jason Wang
>
> v3:
>     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     | 97 +++++++++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 96 insertions(+), 3 deletions(-)

(...)

> @@ -3786,7 +3791,8 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
>  not set VIRTIO_NET_HDR_F_RSC_INFO bit in \field{flags}.
>
>  If one of the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options have
> -been negotiated, the device SHOULD set \field{hdr_len} to a value
> +been negotiated and the VIRTIO_NET_HDR_F_SPLIT_HEADER bit in \field{flags}
> +is not set, the device SHOULD set \field{hdr_len} to a value
>  not less than the length of the headers, including the transport
>  header.

One thing I find a bit hard to figure out is what hdr_len is supposed to
look like if the SPLIT_HEADER flag is actually on. It is explained in
the split header section, but I feel like the reader is left a bit
hanging here.

Not sure how to solve this; maybe simply add a pointer to the split
header specification below?

(...)

> +A device MUST NOT split the header encounter any of the following cases:

Maybe "A device MUST NOT split the header if it encounters any of the
following cases"?

> +\begin{itemize}
> +    \item the device does not recognize the protocol of the packet.
> +    \item the packet is an IP fragmentation.
> +    \item \field{type} does not include the protocol of the packet.
> +    \item the buffer consists of only one descriptor.
> +    \item the total size of the virtio net header and the protocol header exceeds
> +        the size of the first descriptor.
> +    \item when VIRTIO_NET_F_MRG_RXBUF is not negotiated and the size of the
> +        payload exceeds the size of the descriptor chain starting from the 2nd
> +        descriptor.
> +\end{itemize}
> +
> +If the header is split by the device, the VIRTIO_NET_HDR_F_SPLIT_HEADER bit
> +in \field{flags} MUST be set. The protocol header MUST be on the first
> +descriptor, following the virtio net header. The payload MUST start from 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 received 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 first descriptor, because the first descriptor is used to
> +carry the protocol header. The used length still reports the number of bytes it
> +has written to memory.

Personally, I find this paragraph hard to follow... maybe something like
the following:

"If all of the following applies:
\begin{itemize}
    \item the header is split by the device,
    \item VIRTIO_NET_F_MRG_RXBUF has been negotiated,
    \item the received packet is split over multiple buffers,
    \item the device uses a buffer other than the first, [*]
    \item the buffer consists of multiple descriptors
\end {itemize}
the device MUST skip the first descriptor, because the first descriptor
is used to carry the protocol header. The used length still reports the
number of bytes it has written to memory."

Although I'm not quite sure what [*] means, the first of what?