Re: [virtio-dev] [PATCH v8] virtio_net: support for split transport header

[Jason Wang <jasowang@redhat.com>]

On Wed, Sep 21, 2022 at 2:20 PM Jason Wang <jasowang@redhat.com> wrote:
>
> On Tue, Sep 20, 2022 at 11:28 AM Heng Qi <hengqi@linux.alibaba.com> wrote:
> >
> > On Tue, Sep 20, 2022 at 09:59:22AM +0800, Jason Wang wrote:
> > >
> > > å 2022/9/16 10:56, hengqi åé:
> > > >From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > >
> > > >The purpose of this feature is to split the transport header and the payload
> > > >of the packet.
> > > >
> > > >|                     receive buffer1(page)            | receive buffer2(page) |
> > > >|<- offset ->| virtnet hdr | mac | ip | tcp |<- hold ->|        payload        |
> > > >              |<------------------------------->|
> > > >                               ^
> > > >                               |
> > > >                            max_len
> > > >
> > > >We can use one page for every 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>
> > > >---
> > > >v8:
> > > >     1. Do not depend on descriptor chain. @Michael S. Tsirkin
> > > >     2. Add \field{offset} and \field{max_len}.
> > > >     3. Fix some presentation issues. @Jason Wang
> > > >     4. Clarify some paragraphs.
> > > >
> > > >v7:
> > > >     1. Fix some presentation issues.
> > > >     2. Use "split transport header". @Jason Wang
> > > >     3. Clarify some paragraphs. @Cornelia Huck
> > > >     4. determine the device what to do if it does not perform header split on a packet.
> > > >
> > > >v6:
> > > >     1. Fix some syntax issues. @Cornelia Huck
> > > >     2. Clarify some paragraphs. @Cornelia Huck
> > > >     3. Determine the device what to do if it does not perform header split on a packet.
> > > >
> > > >v5:
> > > >     1. Determine when hdr_len is credible in the process of rx
> > > >     2. Clean up the use of buffers and descriptors
> > > >     3. Clarify the meaning of used lenght if the first descriptor is skipped in the case of merge
> > > >
> > > >v4:
> > > >     1. fix typo @Cornelia Huck @Jason Wang
> > > >     2. do not split header for IP fragmentation packet. @Jason Wang
> > > >
> > > >  conformance.tex |  2 ++
> > > >  content.tex     | 91 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > >  2 files changed, 93 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 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 / Notifications Coalescing}
> > > >+\item \ref{drivernormative:Device Types / Network Device / Device Operation / Control Virtqueue / Split Transport Header}
> > > >  \end{itemize}
> > > >  \conformance{\subsection}{Block Driver Conformance}\label{sec:Conformance / 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 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 / Notifications Coalescing}
> > > >+\item \ref{devicenormative:Device Types / Network Device / Device Operation / Control Virtqueue / Split Transport 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 e863709..fad9dea 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.
> > > >+\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.
> > > >  \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_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 and VIRTIO_NET_F_MRG_RXBUF.
> > > >  \end{description}
> > > >  \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 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 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 set,
> > > >+the driver SHOULD treat the \field{hdr_len} as the length of the transport
> > > >+header inside the first buffer.
> > > >+
> > > >  \begin{note}
> > > >  This is due to various bugs in implementations.
> > > >  \end{note}
> > > >@@ -4483,6 +4493,87 @@ \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 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
> > > >+buffers.
> > > >+
> > > >+\subparagraph{Split Transport Header}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Split Transport Header / Setting 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;
> > > >+    le16 offset;
> > > >+    le16 max_len;
> > > >+};
> > > >+
> > > >+#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
> > > >+header configuration.
> > > >+
> > > >+The driver can enable or disable split transport header for different transport
> > > >+protocols 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 Types / Network Device / Device Operation / Control Virtqueue / Split Transport Header}
> > > >+
> > > >+A device MUST disable transport header splitting upon reset and initialization.
> > > >+
> > > >+If VIRTIO_NET_F_SPLIT_TRANSPORT_HEADER is negotiated, the device MUST support
> > > >+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 transport protocol of the packet.
> > > >+    \item The packet is an IP fragmentation.
> > > >+    \item The splitting of the specific transport protocol is not enabled via
> > > >+        VIRTIO_NET_CTRL_SPLIT_TRANSPORT_HEADER_SET.
> > > >+    \item At most one buffer is available.
> > >
> > >
> > > So this means the feature is disabled for the device without
> > > merge-able buffer? Note that, even in the case of mergeable buffer,
> > > it doesn't mean a buffer that only contains a single descriptor.
> > >
> > >
> >
> > Yes, since the purpose of this scheme is to no longer depend on descriptor chains,
> > the buffer submitted to the receiveq can be thought of as containing only one descriptor.
> > So this feature depends on the mergeable buffer.
>
> To tell the truth, I'm not sure this is a good choice. We never had a
> feature that depends solely on the mergeable rx buffer before.
> Especially considering that using a descriptor chain is not hard. And
> I'm not sure we should care too much on the overhead since the
> splitting is enabled by the administrator when it needs e.g zerocopy.
>
> >
> > > >+    \item The total size of the virtio net header and the transport header exceeds \field{max_len}.
> > >
> > >
> > > I don't get the reason why we need max_len. Can't it implied in the
> > > length of the first descriptor?
> > >
> >
> > Split transport header is usually used in high-throughput scenarios, such as GSO-enabled cases.
> > Therefore, it is best to reserve tailroom with $ (the length of the buffer) - (\field{offset} + \filed{max_len}) $
> > in the first buffer to build the non-linear data area of the socket buffer.

But this doesn't answer the question, without max_len, if tailroom is
needed, we can simply allocate a larger buffer?

Thanks

> >
> > >
> > > >+\end{itemize}
> > > >+
> > > >+If the transport 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
> > > >+buffer, following the virtio net header. The payload MUST start from the
> > > >+second buffer. The device MUST set \field{hdr_len} of structure
> > > >+virtio_net_hdr to the length of the transport header.
> > > >+The used length still reports the number of bytes it has written to memory.
> > > >+
> > > >+\field{offset} and \field{max_len} are valid when device uses the first buffer.
> > > >+The device MUST reserve space in the first buffer using \filed{offset}.
> > > >+If \field{offset} exceeds the length of the buffer, the device MUST drop
> > > >+the receive packets.
> > >
> > >
> > > Can the device simply don't split the packet in this case? Anyhow we
> > > need synchronize the driver with the device in the case (e.g when
> > > driver is try to having a new max_len).
> > >
> >
> > We think that \field{offset} is actively set by the driver, so the driver
> > will also receive packets according to this offset.
> > But if the case is considered to be caused by driver error settings,
> > the device can do not split the packet.
>
> Note that protocol like ipv6 allows variable length of the header,
> falling back to not split the header seems better to me.
>
> Thanks
>
> >
> > > (I wonder if the offset deserves a independent feature (but depends
> > > on the merge able) in this case).
> > >
> >
> > Okay, we can consider later.
> >
> > >
> > > >  The maximum available length of the first buffer
> > > >+used by the device is specified by \field{max_len}.
> > >
> > >
> > > Similarly the max length seems to be implied by length - offset?
> > >
> >
> > You can refer to the above answer about \field{max_len} similarly.
> >
> > Thanks.
> >
> >
> > > Thanks
> > >
> > >
> > > >  If \field{max_len} is 0 or
> > > >+$ \field{offset} + \field{max_len} $ is greater than the length of the buffer,
> > > >+the device can use the entire buffer starting at \field{offset}.
> > > >+
> > > >+\drivernormative{\subparagraph}{Setting Split Transport Header}{Device Types / Network Device / Device Operation / Control Virtqueue / Split Transport Header}
> > > >+
> > > >+If the VIRTIO_NET_HDR_F_SPLIT_TRANSPORT_HEADER bit in \field{flags} is set, the driver
> > > >+SHOULD treat the contents of \field{hdr_len} as the length of the transport header
> > > >+inside the first buffer.
> > > >+
> > > >+If \field{max_len} is not equal to 0, it MUST be greater than the size of the virtio net header.
> > > >  \paragraph{Notifications Coalescing}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing}
> >