[PATCH] virtio-net: support distinguishing between partial and full checksum

[Heng Qi <hengqi@linux.alibaba.com>]

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 negotiate the
new feature provided by this patch, then the driver can load XDP.

A new feature bit VIRTIO_NET_F_GUEST_FULL_CSUM is added to solve the above
situation, which forces the device to supply fully checksummed packets
to the driver.

Use case example:
If VIRTIO_NET_F_GUEST_FULL_CSUM is negotiated and 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 with performant devices: e.g. bringing the driver advantages
such as cpu savings.

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>
---
 device-types/net/description.tex | 38 +++++++++++++++++++++++++++-----
 1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/device-types/net/description.tex b/device-types/net/description.tex
index 76585b0..a63d497 100644
--- a/device-types/net/description.tex
+++ b/device-types/net/description.tex
@@ -88,6 +88,8 @@ \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_GUEST_FULL_CSUM (50)] Driver handles packets with full checksum.
+
 \item[VIRTIO_NET_F_HASH_TUNNEL(51)] Device supports inner header hash for encapsulated packets.
 
 \item[VIRTIO_NET_F_VQ_NOTF_COAL(52)] Device supports virtqueue notification coalescing.
@@ -133,6 +135,7 @@ \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.
 
 \item[VIRTIO_NET_F_HOST_TSO4] Requires VIRTIO_NET_F_CSUM.
 \item[VIRTIO_NET_F_HOST_TSO6] Requires VIRTIO_NET_F_CSUM.
@@ -390,6 +393,11 @@ \subsection{Device Initialization}\label{sec:Device Types / Network Device / Dev
 \ref{sec:Device Types / Network Device / Device Operation /
 Processing of Incoming Packets}~\nameref{sec:Device Types /
 Network Device / Device Operation / Processing of Incoming Packets} below.
+
+\item The VIRTIO_NET_F_GUEST_FULL_CSUM feature indicates that the driver handles
+  packets with full checksum and does not handle packets with partial checksum,
+  which means that partially checksummed packets can not be received by the driver
+  and the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} can not be set by the device.
 \end{enumerate}
 
 A truly minimal driver would only accept VIRTIO_NET_F_MAC and ignore
@@ -720,7 +728,8 @@ \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
@@ -744,7 +753,8 @@ \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
@@ -802,8 +812,9 @@ \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
@@ -819,11 +830,24 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
 	fully checksummed packet;
 \end{enumerate}
 
+If the VIRTIO_NET_F_GUEST_FULL_CSUM feature has been negotiated, the device
+MUST NOT set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} and
+MUST supply a fully checksummed packet to the driver.
+If a partially checksummed packet is received by the device, the device MUST
+calculate full checksum for the packet and then supply it to the driver
+\ref{sec:Device Types / Network Device / Device Operation / Packet Transmission}.
+
 If none of the VIRTIO_NET_F_GUEST_TSO4, TSO6, UFO, USO4 or USO6 options have
 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 been negotiated and
+\field{gso_type} differs from VIRTIO_NET_HDR_GSO_NONE, then the device
+MUST NOT set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} and
+MUST calculate full checksum for the packet and then supply it to the driver.
+
+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
@@ -839,7 +863,8 @@ \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
@@ -1624,6 +1649,7 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
 #define VIRTIO_NET_F_GUEST_TSO6       8
 #define VIRTIO_NET_F_GUEST_ECN        9
 #define VIRTIO_NET_F_GUEST_UFO        10
+#define VIRTIO_NET_F_GUEST_FULL_CSUM  50
 #define VIRTIO_NET_F_GUEST_USO4       54
 #define VIRTIO_NET_F_GUEST_USO6       55
 
-- 
2.19.1.6.gb485710b