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

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Date:   Thu, 10 Jun 2021 12:13:46 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Tanner Love <tannerlove.kernel@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Petar Penkov <ppenkov@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Tanner Love <tannerlove@...gle.com>
Subject: Re: [PATCH net-next v4 2/3] virtio_net: add optional flow dissection
 in virtio_net_hdr_to_skb


在 2021/6/10 下午12:05, Alexei Starovoitov 写道:
> On Wed, Jun 9, 2021 at 8:53 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>> It works but it couples virtio with bpf.
> Jason,
>
> I think your main concern is that it makes virtio_net_hdr into uapi?


Actually not. Note that, vnet header is already a part of uAPI 
(include/uapi/linux/virtio_net.h).

The concern is that is may confuse the userspace since we will have two 
sets of vnet header uapi (and they are already out of sync).


> That's not the case. __sk_buff is uapi, but pointers to sockets
> and other kernel data structures are not.
> Yes. It's a bit weird that uapi struct has a pointer to kernel internal,
> but I don't see it as a deal breaker.


Yes, but looking at the existing fields of __sk_buff. All are pretty 
generic fields that are device agnostic. This patch breaks this.


> Tracing progs have plenty of such cases.
> In networking there is tcp-bpf where everything is kernel internal and non-uapi.
> So after this patch virtio_net_hdr is free to change without worrying about bpf
> progs reading it.


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

Thanks