Tue, Feb 21, 2023 at 04:11:53PM CET, willemdebruijn.kernel@xxxxxxxxx wrote: >Jiri Pirko wrote: >> From: Jiri Pirko <jiri@xxxxxxxxxx> >> >> 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@xxxxxxxxxx> >> --- >> 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 Well, the tap driver actually trusts the hdr_len to be of correct header size nowadays. >still need to be careful against malicious drivers, so cannot assume >much beyond what they do today (i.e., a hint). Malicious how? There is upper limit of size in tap which is checked. I assume that for hw implementation, that would be the same. But anyway, this discussion would be rather part of the spec/device patch, don't you think? > >Might be good to point to the definition commit: >https://github.com/oasis-tcs/virtio-spec/commit/4f1981a1ff46b7aeb801c4c524ff76e93d9ce022 There were couple of fixes to the spec since then, that's why I didn't include it. It is trivial to look it up in the spec. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
