Re: [PATCH net-next v4 2/3] virtio_net: add optional flow dissection in virtio_net_hdr_to_skb

On Thu, Jun 10, 2021 at 1:25 AM Jason Wang [off-list ref] wrote:

在 2021/6/10 下午12:19, Alexei Starovoitov 写道:

quoted

On Wed, Jun 9, 2021 at 9:13 PM Jason Wang [off-list ref] wrote:

quoted

So I wonder why not simply use helpers to access the vnet header like
how tcp-bpf access the tcp header?

Short answer - speed.
tcp-bpf accesses all uapi and non-uapi structs directly.

Ok, this makes sense. But instead of coupling device specific stuffs
like vnet header and neediness into general flow_keys as a context.

It would be better to introduce a vnet header context which contains

1) vnet header
2) flow keys
3) other contexts like endian and virtio-net features

So we preserve the performance and decouple the virtio-net stuffs from
general structures like flow_keys or __sk_buff.

You are advocating for a separate BPF program that takes a vnet hdr
and flow_keys as context and is run separately after flow dissection?

I don't understand the benefit of splitting the program in two in this manner.

Your previous comment mentions two vnet_hdr definitions that can get
out of sync. Do you mean v1 of this patch, that adds the individual
fields to bpf_flow_dissector? That is no longer the case: the latest
version directly access the real struct. As Alexei points out, doing
this does not set virtio_net_hdr in stone in the ABI. That is a valid
worry. But so this patch series will not restrict how that struct may
develop over time. A version field allows a BPF program to parse the
different variants of the struct -- in the same manner as other
protocol headers. If you prefer, we can add that field from the start.
I don't see a benefit to an extra layer of indirection in the form of
helper functions.

I do see downsides to splitting the program. The goal is to ensure
consistency between vnet_hdr and packet payload. A program split
limits to checking vnet_hdr against what the flow_keys struct has
extracted. That is a great reduction over full packet access. For
instance, does the packet contain IP options? No idea.

If stable ABI is not a concern and there are no different struct
definitions that can go out of sync, does that address your main
concerns?