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 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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
