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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Jiri Pirko wrote:
> From: Jiri Pirko <jiri@nvidia.com>
> 
> 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 <jiri@nvidia.com>
> ---
> 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