Re: [virtio-comment] [PATCH] virtio-net: Add support for correct hdr_len field.

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

On Thu, Oct 24, 2019 at 03:24:43PM +0000, Vitaly Mireyno wrote:
> Some devices benefit from the knowledge of the exact header length for TSO processing.
> Add a feature bit for a driver that is capable of providing the correct header length for TSO packets.
> 
> Signed-off-by: Vitaly Mireyno <vmireyno@marvell.com>

Actually I was wondering about that. How important is this in practice?
Reason is I am looking to drop fields from the header.

> ---
>  content.tex | 49 +++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 39 insertions(+), 10 deletions(-)
> 
> diff --git a/content.tex b/content.tex
> index 679391e..dac6921 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -2811,6 +2811,9 @@ \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_GUEST_HDRLEN(59)] Driver can provide the exact \field{hdr_len}
> +    value.
> +
>  \item[VIRTIO_NET_F_RSC_EXT(61)] Device can process duplicated ACKs
>      and report number of coalesced segments and duplicated ACKs
>  
> @@ -2840,6 +2843,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
>  \item[VIRTIO_NET_F_MQ] Requires VIRTIO_NET_F_CTRL_VQ.
>  \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_GUEST_HDRLEN] Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_NET_F_HOST_TSO6 or VIRTIO_NET_F_HOST_UFO.
>  \end{description}
>  
>  \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Network Device / Feature bits / Legacy Interface: Feature bits}
> @@ -3095,12 +3099,21 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
>    into smaller packets. The other gso fields are set:
>  
>    \begin{itemize}
> -  \item \field{hdr_len} is a hint to the device as to how much of the header
> +  \item If the VIRTIO_NET_F_GUEST_HDRLEN feature has been negotiated,
> +    \field{hdr_len} indicates the header length that needs to be replicated
> +    for each packet. It's a number of bytes from beginning of the packet
> +    to beginning of the transport payload.
> +    Otherwise, if the VIRTIO_NET_F_GUEST_HDRLEN feature has not been negotiated,
> +    \field{hdr_len} is a hint to the device as to how much of the header
>      needs to be kept to copy into each packet, usually set to the
>      length of the headers, including the transport header\footnote{Due to various bugs in implementations, this field is not useful
>  as a guarantee of the transport header size.
>  }.
>  
> +  \begin{note}
> +  Some devices benefit from the knowledge of the exact header length, for TSO processing.
> +  \end{note}
> +
>    \item \field{gso_size} is the maximum size of each packet beyond that
>      header (ie. MSS).
>  
> @@ -3173,9 +3186,17 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
>  desired MSS.
>  
>  If one of the VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO options have
> -been negotiated, the driver SHOULD set \field{hdr_len} to a value
> -not less than the length of the headers, including the transport
> -header.
> +been negotiated:
> +\begin{itemize}
> +\item If the VIRTIO_NET_F_GUEST_HDRLEN feature has been negotiated,
> +	the driver MUST set \field{hdr_len} to a value equal to the length
> +	of the headers, including the transport header.
> +
> +\item If the VIRTIO_NET_F_GUEST_HDRLEN feature has not been negotiated,
> +	the driver SHOULD set \field{hdr_len} to a value
> +	not less than the length of the headers, including the transport
> +	header.
> +\end{itemize}
>  
>  The driver MUST NOT set the VIRTIO_NET_HDR_F_DATA_VALID and
>  VIRTIO_NET_HDR_F_RSC_INFO bits in \field{flags}.
> @@ -3187,12 +3208,20 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
>  device MUST NOT use the \field{csum_start} and \field{csum_offset}.
>  
>  If one of the VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO options have
> -been negotiated, the device MAY use \field{hdr_len} only as a hint about the
> -transport header size.
> -The device MUST NOT rely on \field{hdr_len} to be correct.
> -\begin{note}
> -This is due to various bugs in implementations.
> -\end{note}
> +been negotiated:
> +\begin{itemize}
> +\item If the VIRTIO_NET_F_GUEST_HDRLEN feature has been negotiated,
> +	the device MAY use \field{hdr_len} as the transport header size.
> +
> +\item If the VIRTIO_NET_F_GUEST_HDRLEN feature has not been negotiated,
> +	the device MAY use \field{hdr_len} only as a hint about the
> +	transport header size.
> +	The device MUST NOT rely on \field{hdr_len} to be correct.
> +
> +	\begin{note}
> +	This is due to various bugs in implementations.
> +	\end{note}
> +\end{itemize}
>  
>  If VIRTIO_NET_HDR_F_NEEDS_CSUM is not set, the device MUST NOT
>  rely on the packet checksum being correct.
> -- 
> 
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