On Tue, Feb 21, 2023 at 04:39:32PM +0100, Jiri Pirko wrote: > 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. This might be a good link: https://docs.oasis-open.org/virtio/virtio/v1.2/cs01/virtio-v1.2-cs01.html#x1-230006x3 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
