From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id 81DE298642E for ; Tue, 30 Aug 2022 11:26:56 +0000 (UTC) Message-ID: <3ebb84aa-d00b-464d-d624-36ae1ec7b285@linux.alibaba.com> Date: Tue, 30 Aug 2022 19:26:50 +0800 MIME-Version: 1.0 From: Heng Qi References: <1660642495-104002-1-git-send-email-hengqi@linux.alibaba.com> In-Reply-To: <1660642495-104002-1-git-send-email-hengqi@linux.alibaba.com> Subject: Re: [virtio-dev] [PATCH v7] virtio_net: support split header Content-Type: multipart/alternative; boundary="------------EjHmAODGuhy0iMqMDABG9in0" To: Jason Wang , mst@redhat.com Cc: virtio-dev@lists.oasis-open.org, xuanzhuo@linux.alibaba.com, kangjie.xu@linux.alibaba.com List-ID: --------------EjHmAODGuhy0iMqMDABG9in0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: quoted-printable =E5=9C=A8 2022/8/16 =E4=B8=8B=E5=8D=885:34, Heng Qi =E5=86=99=E9=81=93: > From: Xuan Zhuo > > 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 > Signed-off-by: Heng Qi > Reviewed-by: Kangjie Xu > --- > v7: > =091. Fix some presentation issues. > =092. Use "split transport header". @Jason Wang > =093. Clarify some paragraphs. @Cornelia Huck > =094. determine the device what to do if it does not perform header split= on a packet. > > v6: > =091. Fix some syntax issues. @Cornelia Huck > =092. Clarify some paragraphs. @Cornelia Huck > =093. Determine the device what to do if it does not perform header split= on a packet. > > v5: > =091. Determine when hdr_len is credible in the process of rx > =092. Clean up the use of buffers and descriptors > =093. Clarify the meaning of used lenght if the first descriptor is skipp= ed in the case of merge > > v4: > =091. fix typo @Cornelia Huck @Jason Wang > =092. do not split header for IP fragmentation packet. @Jason Wang > > v3: > =091. Fix some syntax issues > =092. Fix some terminology issues > =093. It is not unified with ip alignment, so ip alignment is not include= d > =094. Make it clear that the device must support four types, in the case = of successful negotiation. > > conformance.tex | 2 ++ > content.tex | 102 +++++++++++++++++++++++++++++++++++++++++++++++++= +++++++ > 2 files changed, 104 insertions(+) > > diff --git a/conformance.tex b/conformance.tex > index 2b86fc6..4e2b82e 100644 > --- a/conformance.tex > +++ b/conformance.tex > @@ -150,6 +150,7 @@ \section{Conformance Targets}\label{sec:Conformance /= Conformance Targets} > \item \ref{drivernormative:Device Types / Network Device / Device Opera= tion / Control Virtqueue / Offloads State Configuration / Setting Offloads = State} > \item \ref{drivernormative:Device Types / Network Device / Device Opera= tion / Control Virtqueue / Receive-side scaling (RSS) } > \item \ref{drivernormative:Device Types / Network Device / Device Opera= tion / Control Virtqueue / Notifications Coalescing} > +\item \ref{drivernormative:Device Types / Network Device / Device Operat= ion / Control Virtqueue / Split Transport Header} > \end{itemize} > =20 > \conformance{\subsection}{Block Driver Conformance}\label{sec:Conforman= ce / Driver Conformance / Block Driver Conformance} > @@ -415,6 +416,7 @@ \section{Conformance Targets}\label{sec:Conformance /= Conformance Targets} > \item \ref{devicenormative:Device Types / Network Device / Device Opera= tion / Control Virtqueue / Automatic receive steering in multiqueue mode} > \item \ref{devicenormative:Device Types / Network Device / Device Opera= tion / Control Virtqueue / Receive-side scaling (RSS) / RSS processing} > \item \ref{devicenormative:Device Types / Network Device / Device Opera= tion / Control Virtqueue / Notifications Coalescing} > +\item \ref{devicenormative:Device Types / Network Device / Device Operat= ion / Control Virtqueue / Split Transport Header} > \end{itemize} > =20 > \conformance{\subsection}{Block Device Conformance}\label{sec:Conforman= ce / Device Conformance / Block Device Conformance} > diff --git a/content.tex b/content.tex > index e863709..5676da9 100644 > --- a/content.tex > +++ b/content.tex > @@ -3084,6 +3084,9 @@ \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. > =20 > +\item[VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER (52)] Device supports splittin= g > + the transport header and the payload. > + > \item[VIRTIO_NET_F_NOTF_COAL(53)] Device supports notifications coalesc= ing. > =20 > \item[VIRTIO_NET_F_GUEST_USO4 (54)] Driver can receive USOv4 packets. > @@ -3140,6 +3143,7 @@ \subsubsection{Feature bit requirements}\label{sec:= Device Types / Network Device > \item[VIRTIO_NET_F_NOTF_COAL] Requires VIRTIO_NET_F_CTRL_VQ. > \item[VIRTIO_NET_F_RSC_EXT] Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_N= ET_F_HOST_TSO6. > \item[VIRTIO_NET_F_RSS] Requires VIRTIO_NET_F_CTRL_VQ. > +\item[VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER] Requires VIRTIO_NET_F_CTRL_VQ= . > \end{description} > =20 > \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types /= Network Device / Feature bits / Legacy Interface: Feature bits} > @@ -3371,6 +3375,7 @@ \subsection{Device Operation}\label{sec:Device Type= s / 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_TRANSPORT_HEADER 8 > u8 flags; > #define VIRTIO_NET_HDR_GSO_NONE 0 > #define VIRTIO_NET_HDR_GSO_TCPV4 1 > @@ -3823,6 +3828,11 @@ \subsubsection{Processing of Incoming Packets}\lab= el{sec:Device Types / Network > been negotiated, the driver MAY use \field{hdr_len} only as a hint abou= t the > transport header size. > The driver MUST NOT rely on \field{hdr_len} to be correct. > + > +If the VIRTIO_NET_HDR_F_SPLIT_TRANSPORT_HEADER bit in \field{flags} is s= et, > +the driver SHOULD treat the \field{hdr_len} as the length of the transpo= rt header > +inside the first descriptor. > + > \begin{note} > This is due to various bugs in implementations. > \end{note} > @@ -4483,6 +4493,98 @@ \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. > =20 > +\paragraph{Split Transport Header}\label{sec:Device Types / Network Devi= ce / Device Operation / Control Virtqueue / Split Transport Header} > + > +If the VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER feature is negotiated, > +the device supports splitting the transport header and the payload. > +The transport header and the payload will be separated into different > +descriptors. > + > +\subparagraph{Split Transport Header}\label{sec:Device Types / Network D= evice / Device Operation / Control Virtqueue / Split Transport Header / Set= ting Split Transport Header} > + > +To configure the split transport header, the following layout structure = and definitions > +are used: > + > +\begin{lstlisting} > +struct virtio_net_split_transport_header_config { > +#define VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_TCP4 (1 << 0) > +#define VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_TCP6 (1 << 1) > +#define VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_UDP4 (1 << 2) > +#define VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_UDP6 (1 << 3) > + le64 type; > +}; > + > +#define VIRTIO_NET_CTRL_SPLIT_TRANSPORT_HEADER 6 > + #define VIRTIO_NET_CTRL_SPLIT_TRANSPORT_HEADER_SET 0 > +\end{lstlisting} > + > +The class VIRTIO_NET_CTRL_SPLIT_TRANSPORT_HEADER has one command: > +VIRTIO_NET_CTRL_SPLIT_TRANSPORT_HEADER_SET applies the new split transpo= rt header configuration. > + > +The driver can enable or disable split transport header for different pr= otocols by > +setting or clearing corresponding bits in \field{type}. > + > +\begin{itemize} > + \item VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_TCP4: split after ipv4 = tcp header > + \item VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_TCP6: split after ipv6 = tcp header > + \item VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_UDP4: split after ipv4 = udp header > + \item VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_UDP6: split after ipv6 = udp header > +\end{itemize} > + > +\devicenormative{\subparagraph}{Setting Split Transport Header}{Device T= ypes / Network Device / Device Operation / Control Virtqueue / Split Transp= ort Header} > + > +A device MUST initialize \field{type} to 0, and MUST set it to 0 > +upon device reset. > + > +If VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER is negotiated, the device MUST su= pport > +VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_TCP4, VIRTIO_NET_SPLIT_TRANSPORT_= HEADER_TYPE_TCP6, > +VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_UDP4, VIRTIO_NET_SPLIT_TRANSPORT_= HEADER_TYPE_UDP6. > + > +A device MUST NOT split the transport 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 transport head= er 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 t= he 2nd > + descriptor. > +\end{itemize} > + > +If the header is split by the device, the VIRTIO_NET_HDR_F_SPLIT_TRANSPO= RT_HEADER bit > +in \field{flags} MUST be set. The transport 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 transport header. > + > +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 spread over multiple buffers. > +\end {itemize} > +then if the device uses the buffers after the 1st buffer, and the buffer > +consists of multiple descriptors, the device MUST skip the first descrip= tor, > +because the first descriptor is used to carry the transport header. > +The used length still reports the number of bytes it has written to memo= ry. > + > +If VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER is negotiated, but the device doe= s not split the > +virtio net header and the transport header, and the buffer consists of a= t least two > +descriptors, the device MUST start with the first descriptor to store th= e packet, and > +MUST NOT set the VIRTIO_NET_HDR_F_SPLIT_TRANSPORT_HEADER bit in \field{f= lags}. > + > +\drivernormative{\subparagraph}{Setting Split Transport Header}{Device T= ypes / Network Device / Device Operation / Control Virtqueue / Split Transp= ort Header} > + > +If the VIRTIO_NET_HDR_F_SPLIT_TRANSPORT_HEADER bit in \field{flags} is s= et, the driver > +MUST treat the contents of \field{hdr_len} as the length of the transpor= t header > +inside the first descriptor. > + > +If the split transport header is enabled, the buffers submitted to recei= veq by the > +driver MUST be composed of at least two descriptors. > +When the buffer consists of two descriptors, the length of the first > +descriptor MUST be greater than the one of the virtio net header. > =20 > \paragraph{Notifications Coalescing}\label{sec:Device Types / Network D= evice / Device Operation / Control Virtqueue / Notifications Coalescing} > =20 Do you have any comments for this v7 version? --------------EjHmAODGuhy0iMqMDABG9in0 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


=E5=9C=A8 2022/8/16 =E4=B8=8B=E5=8D=885:= 34, Heng Qi =E5=86=99=E9=81=93:
From: Xuan Zhuo <xuanzhu=
o@linux.alibaba.com>

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>
Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Reviewed-by: Kangjie Xu <kangjie.xu@linux.alibaba.com>
---
v7:
=091. Fix some presentation issues.
=092. Use "split transport header". @Jason Wang
=093. Clarify some paragraphs. @Cornelia Huck
=094. determine the device what to do if it does not perform header split o=
n a packet.

v6:
=091. Fix some syntax issues. @Cornelia Huck
=092. Clarify some paragraphs. @Cornelia Huck
=093. Determine the device what to do if it does not perform header split o=
n a packet.

v5:
=091. Determine when hdr_len is credible in the process of rx
=092. Clean up the use of buffers and descriptors
=093. Clarify the meaning of used lenght if the first descriptor is skipped=
 in the case of merge

v4:
=091. fix typo @Cornelia Huck @Jason Wang
=092. do not split header for IP fragmentation packet. @Jason Wang

v3:
=091. Fix some syntax issues
=092. Fix some terminology issues
=093. It is not unified with ip alignment, so ip alignment is not included
=094. Make it clear that the device must support four types, in the case of=
 successful negotiation.

 conformance.tex |   2 ++
 content.tex     | 102 ++++++++++++++++++++++++++++++++++++++++++++++++++++=
++++
 2 files changed, 104 insertions(+)

diff --git a/conformance.tex b/conformance.tex
index 2b86fc6..4e2b82e 100644
--- a/conformance.tex
+++ b/conformance.tex
@@ -150,6 +150,7 @@ \section{Conformance Targets}\label{sec:Conformance / C=
onformance Targets}
 \item \ref{drivernormative:Device Types / Network Device / Device Operatio=
n / Control Virtqueue / Offloads State Configuration / Setting Offloads Sta=
te}
 \item \ref{drivernormative:Device Types / Network Device / Device Operatio=
n / Control Virtqueue / Receive-side scaling (RSS) }
 \item \ref{drivernormative:Device Types / Network Device / Device Operatio=
n / Control Virtqueue / Notifications Coalescing}
+\item \ref{drivernormative:Device Types / Network Device / Device Operatio=
n / Control Virtqueue / Split Transport Header}
 \end{itemize}
=20
 \conformance{\subsection}{Block Driver Conformance}\label{sec:Conformance =
/ Driver Conformance / Block Driver Conformance}
@@ -415,6 +416,7 @@ \section{Conformance Targets}\label{sec:Conformance / C=
onformance Targets}
 \item \ref{devicenormative:Device Types / Network Device / Device Operatio=
n / Control Virtqueue / Automatic receive steering in multiqueue mode}
 \item \ref{devicenormative:Device Types / Network Device / Device Operatio=
n / Control Virtqueue / Receive-side scaling (RSS) / RSS processing}
 \item \ref{devicenormative:Device Types / Network Device / Device Operatio=
n / Control Virtqueue / Notifications Coalescing}
+\item \ref{devicenormative:Device Types / Network Device / Device Operatio=
n / Control Virtqueue / Split Transport Header}
 \end{itemize}
=20
 \conformance{\subsection}{Block Device Conformance}\label{sec:Conformance =
/ Device Conformance / Block Device Conformance}
diff --git a/content.tex b/content.tex
index e863709..5676da9 100644
--- a/content.tex
+++ b/content.tex
@@ -3084,6 +3084,9 @@ \subsection{Feature bits}\label{sec:Device Types / Ne=
twork Device / Feature bits
 \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
     channel.
=20
+\item[VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER (52)] Device supports splitting
+    the transport header and the payload.
+
 \item[VIRTIO_NET_F_NOTF_COAL(53)] Device supports notifications coalescing=
.
=20
 \item[VIRTIO_NET_F_GUEST_USO4 (54)] Driver can receive USOv4 packets.
@@ -3140,6 +3143,7 @@ \subsubsection{Feature bit requirements}\label{sec:De=
vice Types / Network Device
 \item[VIRTIO_NET_F_NOTF_COAL] 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_TRANSPORT_HEADER] Requires VIRTIO_NET_F_CTRL_VQ.
 \end{description}
=20
 \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Ne=
twork Device / Feature bits / Legacy Interface: Feature bits}
@@ -3371,6 +3375,7 @@ \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_TRANSPORT_HEADER  8
         u8 flags;
 #define VIRTIO_NET_HDR_GSO_NONE        0
 #define VIRTIO_NET_HDR_GSO_TCPV4       1
@@ -3823,6 +3828,11 @@ \subsubsection{Processing of Incoming Packets}\label=
{sec:Device Types / Network
 been negotiated, the driver MAY use \field{hdr_len} only as a hint about t=
he
 transport header size.
 The driver MUST NOT rely on \field{hdr_len} to be correct.
+
+If the VIRTIO_NET_HDR_F_SPLIT_TRANSPORT_HEADER bit in \field{flags} is set=
,
+the driver SHOULD treat the \field{hdr_len} as the length of the transport=
 header
+inside the first descriptor.
+
 \begin{note}
 This is due to various bugs in implementations.
 \end{note}
@@ -4483,6 +4493,98 @@ \subsubsection{Control Virtqueue}\label{sec:Device T=
ypes / Network Device / Devi
 according to the native endian of the guest rather than
 (necessarily when not using the legacy interface) little-endian.
=20
+\paragraph{Split Transport Header}\label{sec:Device Types / Network Device=
 / Device Operation / Control Virtqueue / Split Transport Header}
+
+If the VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER feature is negotiated,
+the device supports splitting the transport header and the payload.
+The transport header and the payload will be separated into different
+descriptors.
+
+\subparagraph{Split Transport Header}\label{sec:Device Types / Network Dev=
ice / Device Operation / Control Virtqueue / Split Transport Header / Setti=
ng Split Transport Header}
+
+To configure the split transport header, the following layout structure an=
d definitions
+are used:
+
+\begin{lstlisting}
+struct virtio_net_split_transport_header_config {
+#define VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_TCP4     (1 << 0)
+#define VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_TCP6     (1 << 1)
+#define VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_UDP4     (1 << 2)
+#define VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_UDP6     (1 << 3)
+    le64 type;
+};
+
+#define VIRTIO_NET_CTRL_SPLIT_TRANSPORT_HEADER       6
+ #define VIRTIO_NET_CTRL_SPLIT_TRANSPORT_HEADER_SET   0
+\end{lstlisting}
+
+The class VIRTIO_NET_CTRL_SPLIT_TRANSPORT_HEADER has one command:
+VIRTIO_NET_CTRL_SPLIT_TRANSPORT_HEADER_SET applies the new split transport=
 header configuration.
+
+The driver can enable or disable split transport header for different prot=
ocols by
+setting or clearing corresponding bits in \field{type}.
+
+\begin{itemize}
+    \item VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_TCP4: split after ipv4 tc=
p header
+    \item VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_TCP6: split after ipv6 tc=
p header
+    \item VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_UDP4: split after ipv4 ud=
p header
+    \item VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_UDP6: split after ipv6 ud=
p header
+\end{itemize}
+
+\devicenormative{\subparagraph}{Setting Split Transport Header}{Device Typ=
es / Network Device / Device Operation / Control Virtqueue / Split Transpor=
t Header}
+
+A device MUST initialize \field{type} to 0, and MUST set it to 0
+upon device reset.
+
+If VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER is negotiated, the device MUST supp=
ort
+VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_TCP4, VIRTIO_NET_SPLIT_TRANSPORT_HE=
ADER_TYPE_TCP6,
+VIRTIO_NET_SPLIT_TRANSPORT_HEADER_TYPE_UDP4, VIRTIO_NET_SPLIT_TRANSPORT_HE=
ADER_TYPE_UDP6.
+
+A device MUST NOT split the transport header if it encounters any of the f=
ollowing 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 transport header=
 exceeds
+        the size of the first descriptor.
+    \item when VIRTIO_NET_F_MRG_RXBUF is not negotiated and the size of th=
e
+        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_TRANSPORT=
_HEADER bit
+in \field{flags} MUST be set. The transport header MUST be on the first
+descriptor, following the virtio net header. The payload MUST start from t=
he
+second descriptor. The device MUST set \field{hdr_len} of structure
+virtio_net_hdr to the length of the transport header.
+
+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 spread over multiple buffers.
+\end {itemize}
+then if the device uses the buffers after the 1st buffer, and the buffer
+consists of multiple descriptors, the device MUST skip the first descripto=
r,
+because the first descriptor is used to carry the transport header.
+The used length still reports the number of bytes it has written to memory=
.
+
+If VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER is negotiated, but the device does =
not split the
+virtio net header and the transport header, and the buffer consists of at =
least two
+descriptors, the device MUST start with the first descriptor to store the =
packet, and
+MUST NOT set the VIRTIO_NET_HDR_F_SPLIT_TRANSPORT_HEADER bit in \field{fla=
gs}.
+
+\drivernormative{\subparagraph}{Setting Split Transport Header}{Device Typ=
es / Network Device / Device Operation / Control Virtqueue / Split Transpor=
t Header}
+
+If the VIRTIO_NET_HDR_F_SPLIT_TRANSPORT_HEADER bit in \field{flags} is set=
, the driver
+MUST treat the contents of \field{hdr_len} as the length of the transport =
header
+inside the first descriptor.
+
+If the split transport header is enabled, the buffers submitted to receive=
q by the
+driver MUST be composed of at least two descriptors.
+When the buffer consists of two descriptors, the length of the first
+descriptor MUST be greater than the one of the virtio net header.
=20
 \paragraph{Notifications Coalescing}\label{sec:Device Types / Network Devi=
ce / Device Operation / Control Virtqueue / Notifications Coalescing}
=20

Do you have any comments for this v7 version?
--------------EjHmAODGuhy0iMqMDABG9in0--