virtio-net: correct conditions for devices to validate packet checksum

[hengqiali]

There is a historical error in virtio spec:
"If VIRTIO_NET_F_GUEST_CSUM is not negotiated, the device MUST set flags
to zero and SHOULD supply a fully checksummed packet to the driver."

Currently in Linux and virtio-related implementations, the device validates
the packet checksum and sets DATA_VALID regardless of whether
VIRTIO_NET_F_GUEST_CSUM is negotiated.

Please refer to the following summary for details and reasons:

Summary

1. GUEST_CSUM at virtio spec 0.95 is intended to be compatible with partially
   checksummed packets (NEEDS_CSUM <-> CHECKSUM_PARTIAL). So GUEST_CSUM is mapped
   to NETIF_F_RXCSUM.
   GUEST_CSUM only indicates whether the driver handles partially checksummed packets.
   When XDP is loaded, the GUEST_CSUM's offload will be disabled, which means that
   packets have NEEDS_CSUM set will be dropped, and packets have DATA_VALID set will
   still be received.

2. When DATA_VALID was added to Linux in 2011[2] and virtio1.0, it was actually
   expected that rx checksum offload (the driver can set CHECKSUM_UNNECESSARY) had
   nothing to do with whether GUEST_CSUM was negotiated. But due to an error, below
   desctiption was added incorrectly:
   "If VIRTIO_NET_F_GUEST_CSUM is not negotiated, the device MUST set flags
   to zero and SHOULD supply a fully checksummed packet to the driver."

3. We now hope to correct this error. Let the setting of DATA_VALID not be
   controlled by whether GUEST_CSUM is negotiated, but only controlled by whether
   rx checksum offload is enabled on the OS side. The state of this rx checksum
   offload is not aware of the device.

4. [Optional] NETIF_F_RXCSUM corresponding to rx checksum offload should be
   added to dev->hw_features. When the user turns off rx checksum offload through
   ethtool -K, neither NEEDS_CSUM nor DATA_VALID should be taken care of, that is,
   all packets will be marked as CHECKSUM_NONE.

Solution: https://lists.oasis-open.org/archives/virtio-comment/202401/msg00011.html
(This patch already has Jason and Xuan's R-B )

[hengqiali]

This patch already has Jason and Xuan's R-B[1][2], and Huck has said that there is no need to post a new version to carry their R-B tags, so I ask for a friendly vote.
(If you think a new version is needed, I can re-post V9.)

[1] Jason's R-B link: https://lists.oasis-open.org/archives/virtio-comment/202401/msg00020.html
[2] Xuan's R-B link: https://lists.oasis-open.org/archives/virtio-comment/202401/msg00072.html

[mstsirkin]

It's ok but please edit the issue description making sure the first link included is the
proposal patchset. It is unclear which of the links is the proposal.

[hengqiali]

It's ok but please edit the issue description making sure the first link included is the proposal patchset. It is unclear which of the links is the proposal.

Fixed.

[mstsirkin]

BALLOT CREATED AT URL: https://groups.oasis-open.org/higherlogic/ws/public/ballot?id=3835