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Subject: Re: [virtio-dev] Re: [PATCH v2] virtio_net: support split header
On Mon, May 30, 2022 at 01:56:06PM +0800, Jason Wang wrote:
> On Thu, May 26, 2022 at 2:31 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >
> > On Mon, 9 May 2022 16:41:11 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > >
> > > å 2022/5/7 15:15, Xuan Zhuo åé:
> > > > 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>
> > > > ---
> > > > 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.
> > >
> > >
> > > I think you meant "descriptors" instead of "buffers".
> >
> > Will fix.
No, actually, I think this is exactly buffers.
device is always in terms of buffers, descriptors is
a means to implement buffers.
And the feature should
depend on mergeable buffers. Without mergeable buffers you do not
need these tricks, just add two descriptors and be done
with it.
Right?
> >
> > >
> > >
> > > > +
> > > > +\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
> > >
> > >
> > > I think it's better to capitalize: TCPV4 or TCP4.
> >
> > OK.
> >
> > >
> > >
> > > > +#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;
> > > > +};
> > > > +
> > > > +#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.
> > >
> > >
> > > (Not a native speaker, try my best to help)
> > >
> > > This sentence might need some tweaks:
> > >
> > > The driver can enable or disable the split by setting or clearing
> > > corresponding bits in \field{type}.
> >
> > Thanks.
> >
> > >
> > >
> > > > +
> > > > +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.
> > >
> > >
> > > I think it's better to either explain each type one by one instead of
> > > only describing the TCPv4. Or we can simply say the head contains all
> > > headers before level 4 payload?
> >
> > OK. Thanks.
>
So this always splits out at protocol level?
What if device wants to align e.g. IP protocol header?
Ethernet?
Also, what about aligning at an offset?
E.g. NET_IP_ALIGN?
> >
> > >
> > >
> > > > +
> > > > +\devicenormative{\subparagraph}{Setting Split Header}{Device Types / Network Device / Device Operation / Control Virtqueue / Split Header}
> > > > +
> > > > +Split header MUST be disabled after device initialization.
> > >
> > >
> > > I think you actually mean "upon device reset"?
> >
> > Yes.
> >
> >
> > >
> > >
> > > > +
> > > > +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.
> > >
> > >
> > > What's the reason for this limit?
> >
> > Rethinking this problem, some implementation difficulties that I originally
> > thought can be solved. So I think this limitation can be removed.
> >
> > >
> > >
> > > > + \item the receive buffer consists of only one descriptor.
> > >
> > >
> > > descriptor actually.
> >
> > Will fix.
> >
> > >
> > >
> > > > + \item the header exceeds the size of the 0th descriptor.
> > >
> > >
> > > It looks to me the spec count from the first.
> >
> > Will fix.
> >
> > >
> > >
> > > > + \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.
> > >
> > >
> > > Maybe something like the following is better:
> > >
> > > The size of the payload exceeds the length of the descriptor or buffer
> > > chain starting from the 2nd descriptor.
> >
> > OK.
> >
> > >
> > >
> > > > +\end{itemize}
> > > > +
> > > > +If the split header completed,
> > >
> > >
> > > If the header is split by the device?
> >
> > Yes.
> >
> > >
> > >
> > > > then the \field{flags} of virtnet hdr MUST
> > >
> > >
> > > We can drop "then" here and it's better to use structure virtio_net_hdr
> >
> > Will fix.
> >
> > >
> > >
> > > > +contains VIRTIO_NET_HDR_F_SPLIT_HEADER. The header MUST is on the buffer of the
> > > > +0th descriptor,
> > >
> > >
> > > first and second should be used here.
> >
> > OK
> >
> > >
> > >
> > > > and the payload MUST starts from the buffer of the 1th descriptor.
> > > > +The device MUST set \field{hdr_len} of virtnet hdr.
> > >
> > >
> > > To what value?
> >
> > I'll make it clear in the next version.
> >
> > >
> > >
> > > > +
> > > > +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.
> > >
> > >
> > > I don't see why we need this.
> >
> > If used to implement zerocopy, the second desc points to a page-aligned buffer.
> > The first desc points to a small buffer for saving the header. The first buffer
> > and subsequent buffers are generally discontinuous. So giving up the first
> > buffer and directly guaranteeing the data to the page-aligned buffer can achieve
> > better performance. And the driver can reuse the first buffer.
>
> I see, it might be better if we can explain this here.
>
> Thanks
>
> >
> > >
> > >
> > > > +
> > > > +\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}.
> > >
> > >
> > > See above we can reuse the device normative an drop the second part
> > > starting from " the length of the header ..."
> >
> > OK
> >
> > >
> > >
> > > > +
> > > > +If the split header function is enable, the buffers submitted by the driver
> > > > +SHOULD at least be composed of two descriptors.
> > >
> > >
> > > Do we need to mention it's only used for RX? (I think we don't need this
> > > for TX).
> >
> > Yes.
> >
> > >
> > >
> > > > The buffer specified by the 0th
> > > > +descriptor SHOULD be able to accommodate the header.
> > >
> > >
> > > We probably need to clarify that the header here means both the virtio
> > > net header and the protocol header.
> >
> > Will fix.
> >
> > Thanks.
> >
> > >
> > > Thanks
> > >
> > >
> > > >
> > > > \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
> > > > Types / Network Device / Legacy Interface: Framing Requirements}
> > >
> > >
> > > ---------------------------------------------------------------------
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> > > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org
> > >
> >
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