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Message-ID: <64dce2f2b99f5_23f1f82949f@willemb.c.googlers.com.notmuch>
Date: Wed, 16 Aug 2023 10:53:38 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Feng Liu <feliu@...dia.com>,
Willem de Bruijn <willemdebruijn.kernel@...il.com>,
Simon Horman <horms@...nel.org>
Cc: virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
"Michael S . Tsirkin" <mst@...hat.com>,
Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
Bodong Wang <bodong@...dia.com>, Jiri Pirko <jiri@...dia.com>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net v1] virtio_net: Introduce skb_vnet_common_hdr to avoid
typecasting
> >
> > Since legacy virtio will no longer be modified, I don't think there is
> > much value is exposing this new union as UAPI. I do appreciate the
> > benefit to the implementation.
> >
> > [1] https://patches.linaro.org/project/netdev/patch/20210208185558.995292-3-willemdebruijn.kernel@gmail.com/
> Hi, William and Simon
>
> 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.
> 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.
> 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?
This does seem mostly a helper to simplify kernel logic to me, which
is better kept in non-UAPI headers.
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