Re: [PATCH net v1] virtio_net: Introduce skb_vnet_common_hdr to avoid typecasting

[Feng Liu]

On 2023-08-16 a.m.10:53, Willem de Bruijn wrote:
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>
>
> > Thanks for the detailed explanation.
> >
> > I kept virtio_net_hdr_mrg_rxbuf and virtio_net_hdr_v1_hash structures in
> > virtio_net.h, which can be forward compatible with existing user
> > applications which use these structures.
>
> They're UAPI, so we cannot modify or remove them anyway.
>
> Which is exactly why we want to be careful with adding anything new.
ok

> > virtio_net_hdr_v1_hash cannot use virtio_net_hdr as the first member,
> > because in virtio_net_hdr_v1, csum_start and csum_offset are stored in
> > union as a structure, and virtio_net_hdr cannot be used instead.
>
> Oh right. That wasn't always the case, or the reason for this.
> Not super relevant but, commit ed9ecb0415b9 has the history
>
>      virtio: Don't expose legacy net features when VIRTIO_NET_NO_LEGACY defined.
>
>      In particular, the virtio header always has the u16 num_buffers field.
>      We define a new 'struct virtio_net_hdr_v1' for this (rather than
>      simply calling it 'struct virtio_net_hdr', to avoid nasty type errors
>      if some parts of a project define VIRTIO_NET_NO_LEGACY and some don't.
>
>      Transitional devices (which can't define VIRTIO_NET_NO_LEGACY) will
>      have to keep using struct virtio_net_hdr_mrg_rxbuf, which has the same
>      byte layout as struct virtio_net_hdr_v1.
>
> The union was added to overload csum use on tx with RSC use on rx, in
> commit 22b436c9b568. I don't quite follow why there now are three
> structs, rather than two. The first two seem to both implement csum
> partial. Anyway, not super important here.
> ok

> > In addition, I put this new structure virtio_net_common_hdr in uapi,
> > hoping it could be used in future user space application to avoid
> > potential risks caused by type coercion (such as the problems mentioned
> > in the patch description ). So I think it should be in this header file.
> > What do you think?
>
> Adding anything to UAPI has a high bar. Do you have a concrete use
> case for this?

In the scene of with and without VIRTIO_NET_F_HASH_REPORT feature, this 
patch has been tested on my setup, and the function is ok.

> This does seem mostly a helper to simplify kernel logic to me, which
> is better kept in non-UAPI headers.
OK, will change it.