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Subject: [PATCH v4 2/2] virtio-net: support distinguishing between partial and full checksum
virtio-net works in a virtualized system and is somewhat different from
physical nics. One of the differences is that to save virtio device
resources, rx may receive packets with partial checksum. However, XDP may
cause partially checksummed packets to be dropped. So XDP loading conflicts
with the feature VIRTIO_NET_F_GUEST_CSUM.
This patch lets the device to supply fully checksummed packets to the driver.
Then XDP can coexist with VIRTIO_NET_F_GUEST_CSUM to enjoy the benefits of
device verification checksum.
In addition, implementation of some performant devices do not generate
partially checksummed packets, but the standard driver still need to clear
VIRTIO_NET_F_GUEST_CSUM when loading XDP. If these devices enable the
full checksum offloading, then the driver can load XDP without clearing
VIRTIO_NET_F_GUEST_CSUM.
A new feature bit VIRTIO_NET_F_GUEST_FULL_CSUM is added to solve the above
situation, which provides the driver with configurable receive full checksum
offload. If the offload is enabled, then the device must supply fully
checksummed packets to the driver.
Use case example:
If VIRTIO_NET_F_GUEST_FULL_CSUM is negotiated and receive full checksum
offload is enabled, after XDP processes a packet with full checksum, the
VIRTIO_NET_HDR_F_DATA_VALID bit is still retained, resulting in the stack
not needing to validate the checksum again. This is useful for guests:
1. Bring the driver advantages such as cpu savings.
2. For devices that do not generate partially checksummed packets themselves,
XDP can be loaded in the driver without modifying the hardware behavior.
Several solutions have been discussed in the previous proposal[1].
After historical discussion, we have tried the method proposed by Jason[2],
but some complex scenarios and challenges are difficult to deal with.
We now return to the method suggested in [1].
[1] https://lists.oasis-open.org/archives/virtio-dev/202305/msg00291.html
[2] https://lore.kernel.org/all/20230628030506.2213-1-hengqi@linux.alibaba.com/
Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
v3->v4:
- Streamline some repetitive descriptions. @Jason
- Add how features should work, when to be enabled, and overhead. @Jason @Michael
v2->v3:
- Add a section named "Driver Handles Fully Checksummed Packets"
and more descriptions. @Michael
v1->v2:
- Modify full checksum functionality as a configurable offload
that is initially turned off. @Jason
device-types/net/description.tex | 66 ++++++++++++++++++++++---
device-types/net/device-conformance.tex | 1 +
device-types/net/driver-conformance.tex | 1 +
introduction.tex | 3 ++
4 files changed, 65 insertions(+), 6 deletions(-)
diff --git a/device-types/net/description.tex b/device-types/net/description.tex
index 529f470..d08c31e 100644
--- a/device-types/net/description.tex
+++ b/device-types/net/description.tex
@@ -122,6 +122,8 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
device with the same MAC address.
\item[VIRTIO_NET_F_SPEED_DUPLEX(63)] Device reports speed and duplex.
+
+\item[VIRTIO_NET_F_GUEST_FULL_CSUM (64)] Driver handles packets with full checksum.
\end{description}
\subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device / Feature bits / Feature bit requirements}
@@ -136,6 +138,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
\item[VIRTIO_NET_F_GUEST_UFO] Requires VIRTIO_NET_F_GUEST_CSUM.
\item[VIRTIO_NET_F_GUEST_USO4] Requires VIRTIO_NET_F_GUEST_CSUM.
\item[VIRTIO_NET_F_GUEST_USO6] Requires VIRTIO_NET_F_GUEST_CSUM.
+\item[VIRTIO_NET_F_GUEST_FULL_CSUM] Requires VIRTIO_NET_F_GUEST_CSUM and VIRTIO_NET_F_CTRL_GUEST_OFFLOADS.
\item[VIRTIO_NET_F_HOST_TSO4] Requires VIRTIO_NET_F_CSUM.
\item[VIRTIO_NET_F_HOST_TSO6] Requires VIRTIO_NET_F_CSUM.
@@ -398,6 +401,51 @@ \subsection{Device Initialization}\label{sec:Device Types / Network Device / Dev
A truly minimal driver would only accept VIRTIO_NET_F_MAC and ignore
everything else.
+\subsubsection{Driver Handles Fully Checksummed Packets}\label{sec:Device Types / Network Device / Device Initialization / Driver Handles Fully Checksummed Packets}
+
+The VIRTIO_NET_F_GUEST_CSUM feature indicates that the driver can handle
+partially or fully checksummed packets from the device.
+
+When the driver only expects fully checksummed packets, the
+VIRTIO_NET_F_GUEST_FULL_CSUM feature can be negotiated if the device offers it.
+Then the driver only handles packets with full checksum.
+
+If the VIRTIO_NET_F_GUEST_FULL_CSUM feature is negotiated, the driver can
+benefit from the device's ability to calculate and validate the checksum.
+
+Delivering fully checksummed packets rather than partially
+checksummed packets incurs additional overhead for the device.
+The overhead varies from device to device, for example the overhead of
+calculating and validating the packet checksum is a few microseconds
+for a hardware device.
+
+The VIRTIO_NET_F_GUEST_FULL_CSUM feature has a corresponding offload \ref{sec:Device Types /
+Network Device / Device Operation / Control Virtqueue / Offloads State Configuration},
+which when enabled means that the driver only processes packets with full checksum.
+The offload is disabled by default.
+
+The driver can enable the offload by sending the
+VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET command with the
+VIRTIO_NET_F_GUEST_FULL_CSUM bit set when, for example,
+eXpress Data Path (XDP) \hyperref[intro:xdp]{[XDP]} is functioning.
+
+\drivernormative{\subsubsection}{Driver Handles Fully Checksummed Packets}{sec:Device Types / Network Device / Device Initialization / Driver Handles Fully Checksummed Packets}
+
+The driver MUST NOT enable the offload for which VIRTIO_NET_F_GUEST_FULL_CSUM has not been negotiated.
+
+\devicenormative{\subsubsection}{Driver Handles Fully Checksummed Packets}{sec:Device Types / Network Device / Device Initialization / Driver Handles Fully Checksummed Packets}
+
+Upon the device reset, the device MUST disable the offload.
+
+If the offload was enabled, the device behaves as follows:
+\begin{itemize}
+\item The device MUST supply a fully checksummed packet to the driver.
+\item The device MUST NOT set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags}.
+\item The device MAY set the VIRTIO_NET_HDR_F_DATA_VALID bit in \field{flags}, if so,
+the device MUST validate the packet checksum (in case of multiple encapsulated protocols,
+one level of checksums is validated).
+\end{itemize}
+
\subsection{Device Operation}\label{sec:Device Types / Network Device / Device Operation}
Packets are transmitted by placing them in the
@@ -723,7 +771,8 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
\field{num_buffers} is one, then the entire packet will be
contained within this buffer, immediately following the struct
virtio_net_hdr.
-\item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
+\item If the VIRTIO_NET_F_GUEST_CSUM feature (regardless of whether
+ VIRTIO_NET_F_GUEST_FULL_CSUM was negotiated) was negotiated, the
VIRTIO_NET_HDR_F_DATA_VALID bit in \field{flags} can be
set: if so, device has validated the packet checksum.
In case of multiple encapsulated protocols, one level of checksums
@@ -747,7 +796,8 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
number of coalesced TCP segments in \field{csum_start} field and
number of duplicated ACK segments in \field{csum_offset} field
and sets bit VIRTIO_NET_HDR_F_RSC_INFO in \field{flags}.
-\item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
+\item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated but the
+ VIRTIO_NET_F_GUEST_FULL_CSUM feature was not negotiated, the
VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} can be
set: if so, the packet checksum at offset \field{csum_offset}
from \field{csum_start} and any preceding checksums
@@ -805,8 +855,9 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in
\field{gso_type}.
-If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the
-device MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in
+If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated but
+the VIRTIO_NET_F_GUEST_FULL_CSUM feature has not been negotiated,
+the device MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in
\field{flags}, if so:
\begin{enumerate}
\item the device MUST validate the packet checksum at
@@ -826,7 +877,8 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
been negotiated, the device MUST set \field{gso_type} to
VIRTIO_NET_HDR_GSO_NONE.
-If \field{gso_type} differs from VIRTIO_NET_HDR_GSO_NONE, then
+If the VIRTIO_NET_F_GUEST_FULL_CSUM feature has not been negotiated and
+\field{gso_type} differs from VIRTIO_NET_HDR_GSO_NONE, then
the device MUST also set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in
\field{flags} MUST set \field{gso_size} to indicate the desired MSS.
If VIRTIO_NET_F_RSC_EXT was negotiated, the device MUST also
@@ -842,7 +894,8 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
not less than the length of the headers, including the transport
header.
-If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the
+If the VIRTIO_NET_F_GUEST_CSUM feature (regardless of whether
+VIRTIO_NET_F_GUEST_FULL_CSUM has been negotiated) has been negotiated, the
device MAY set the VIRTIO_NET_HDR_F_DATA_VALID bit in
\field{flags}, if so, the device MUST validate the packet
checksum (in case of multiple encapsulated protocols, one level
@@ -1633,6 +1686,7 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
#define VIRTIO_NET_F_GUEST_UFO 10
#define VIRTIO_NET_F_GUEST_USO4 54
#define VIRTIO_NET_F_GUEST_USO6 55
+#define VIRTIO_NET_F_GUEST_FULL_CSUM 64
#define VIRTIO_NET_CTRL_GUEST_OFFLOADS 5
#define VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET 0
diff --git a/device-types/net/device-conformance.tex b/device-types/net/device-conformance.tex
index 52526e4..e72cb5b 100644
--- a/device-types/net/device-conformance.tex
+++ b/device-types/net/device-conformance.tex
@@ -16,4 +16,5 @@
\item \ref{devicenormative:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing}
\item \ref{devicenormative:Device Types / Network Device / Device Operation / Control Virtqueue / Inner Header Hash}
\item \ref{devicenormative:Device Types / Network Device / Device Operation / Control Virtqueue / Device Statistics}
+\item \ref{devicenormative:Device Types / Network Device / Device Initialization / Driver Handles Fully Checksummed Packets}
\end{itemize}
diff --git a/device-types/net/driver-conformance.tex b/device-types/net/driver-conformance.tex
index c693c4f..6a1d7a7 100644
--- a/device-types/net/driver-conformance.tex
+++ b/device-types/net/driver-conformance.tex
@@ -16,4 +16,5 @@
\item \ref{drivernormative:Device Types / Network Device / Device Operation / Control Virtqueue / Notifications Coalescing}
\item \ref{drivernormative:Device Types / Network Device / Device Operation / Control Virtqueue / Inner Header Hash}
\item \ref{drivernormative:Device Types / Network Device / Device Operation / Control Virtqueue / Device Statistics}
+\item \ref{drivernormative:Device Types / Network Device / Device Initialization / Driver Handles Fully Checksummed Packets}
\end{itemize}
diff --git a/introduction.tex b/introduction.tex
index cfa6633..fc99597 100644
--- a/introduction.tex
+++ b/introduction.tex
@@ -145,6 +145,9 @@ \section{Normative References}\label{sec:Normative References}
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP
14, RFC 8174, DOI 10.17487/RFC8174, May 2017
\newline\url{http://www.ietf.org/rfc/rfc8174.txt}\\
+ \phantomsection\label{intro:xdp}\textbf{[XDP]} &
+ eXpress Data Path(XDP) provides a high performance, programmable network data path in the Linux kernel.
+ \newline\url{https://prototype-kernel.readthedocs.io/en/latest/networking/XDP/}\\
\end{longtable}
\section{Non-Normative References}
--
2.19.1.6.gb485710b
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