RE: [patch net-next v2] net: virtio_net: implement exact header length guest feature

Jiri Pirko wrote:

From: Jiri Pirko <redacted>

Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when
set implicates that the driver provides the exact size of the header.

Quoting the original virtio spec:
"hdr_len is a hint to the device as to how much of the header needs to
 be kept to copy into each packet"

"a hint" might not be clear for the reader what does it mean, if it is
"maybe like that" of "exactly like that". This feature just makes it
crystal clear and let the device count on the hdr_len being filled up
by the exact length of header.

Also note the spec already has following note about hdr_len:
"Due to various bugs in implementations, this field is not useful
 as a guarantee of the transport header size."

Without this feature the device needs to parse the header in core
data path handling. Accurate information helps the device to eliminate
such header parsing and directly use the hardware accelerators
for GSO operation.

virtio_net_hdr_from_skb() fills up hdr_len to skb_headlen(skb).
The driver already complies to fill the correct value. Introduce the
feature and advertise it.

Note that virtio spec also includes following note for device
implementation:
"Caution should be taken by the implementation so as to prevent
 a malicious driver from attacking the device by setting
 an incorrect hdr_len."

There is a plan to support this feature in our emulated device.
A device of SolidRun offers this feature bit. They claim this feature
will save the device a few cycles for every GSO packet.

Signed-off-by: Jiri Pirko <redacted>
---
v1->v2:
- extended patch description

Is the expectation that in-kernel devices support this feature, and
if so how would it affect them? If I read the spec correctly, devices
still need to be careful against malicious drivers, so cannot assume
much beyond what they do today (i.e., a hint).

Might be good to point to the definition commit:
https://github.com/oasis-tcs/virtio-spec/commit/4f1981a1ff46b7aeb801c4c524ff76e93d9ce022