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

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

On Tue, May 31, 2022 at 10:10:52AM +0800, Xuan Zhuo wrote:
> On Mon, 30 May 2022 14:27:26 -0400, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > 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.
> 
> Very happy to do this job.
> 
> I thinks "offset" can contain these requirements.

I am not so sure. The issue is with all of the extension headers where
the payload is is not always predictable, even for a
given type.

> No matter how many descriptors
> are used, we treat multiple buffers as a continuous piece of memory. The device
> places ip, tcp/udp at the specified location according to the specified offset.
> 
> The device does not care whether the data is finally aligned with the buffer
> specified by a descriptor, the device only needs to ensure that each layer of
> the protocol is placed on the specified "offset".
> 
> Many other places need to be modified, and I will publish a new spec.
> 
> Thanks.
> 
> >
> > > ---
> > >  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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