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Subject: [PATCH v2] virtio-net: Add support for correct hdr_len field.
Some devices benefit from knowledge of the exact header length. Add a feature bit for a driver that is capable of providing the correct header length. Changes from v1: * Added VIRTIO_NET_F_GUEST_HDRLEN feature offering and acceptance notes * Removed VIRTIO_NET_F_GUEST_HDRLEN dependency on TSO/UFO feature bits Signed-off-by: Vitaly Mireyno <vmireyno@marvell.com> --- content.tex | 51 +++++++++++++++++++++++++++++++++++++++++---------- 1 file changed, 41 insertions(+), 10 deletions(-) diff --git a/content.tex b/content.tex index 679391e..7b52583 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. Device benefits from knowing the exact header length. + \item[VIRTIO_NET_F_RSC_EXT(61)] Device can process duplicated ACKs and report number of coalesced segments and duplicated ACKs @@ -3095,12 +3098,21 @@ \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 knowledge of the exact header length. + \end{note} + \item \field{gso_size} is the maximum size of each packet beyond that header (ie. MSS). @@ -3173,9 +3185,20 @@ \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 SHOULD accept the VIRTIO_NET_F_GUEST_HDRLEN feature if it has +been offered, and if it's able to provide the exact header length. 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 +3210,20 @@ \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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