Re: [PATCH RFC v5 04/10] tun: Unify vnet implementation

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Akihiko Odaki wrote:
> On 2024/10/09 22:55, Willem de Bruijn wrote:
> > Akihiko Odaki wrote:
> >> Both tun and tap exposes the same set of virtio-net-related features.
> >> Unify their implementations to ease future changes.
> >>
> >> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> >> ---
> >>   MAINTAINERS            |   1 +
> >>   drivers/net/tap.c      | 172 ++++++----------------------------------
> >>   drivers/net/tun.c      | 208 ++++++++-----------------------------------------
> >>   drivers/net/tun_vnet.h | 181 ++++++++++++++++++++++++++++++++++++++++++
> > 
> > Same point: should not be in a header.
> > 
> > Also: I've looked into deduplicating code between the various tun, tap
> > and packet socket code as well.
> > 
> > In general it's a good idea. The main counter arguments is that such a
> > break in continuity also breaks backporting fixes to stable. So the
> > benefit must outweight that cost.
> > 
> > In this case, the benefits in terms of LoC are rather modest. Not sure
> > it's worth it.
> > 
> > Even more importantly: are the two code paths that you deduplicate
> > exactly identical? Often in the past the two subtly diverged over
> > time, e.g., due to new features added only to one of the two.
> 
> I find extracting the virtio_net-related code into functions is 
> beneficial.

I understand the benefits. But as I pointed out, it is a trade-off,
not an unconditional good.

> For example, tun_get_user() is a big function and extracting 
> the virtio_net-related code into tun_vnet_hdr_get() will ease 
> understanding tun_get_user() when you are not interested in virtio_net. 
> If virtio_net is your interest, you can look at this group of functions 
> to figure out how they interact with each other.
> 
> Currently, the extracted code is almost identical for tun and tap so 

Almost here is the scary part. Any code that is not exactly identical
must be called out in the commit message.

What often happened is that one of the two got an improvement, whether
fix or feature extension. Then it is likely acceptable to extend this
to the other, too. But we have to review each case.

> they can share it. We can copy the code back (but keep functions as 
> semantic units) if they diverge in the future.

There hopefully is no need to diverge in the future. I suspect that
all previous divergences are accidental.

> > 
> > If so, call out any behavioral changes to either as a result of
> > deduplicating explicitly.
> 
> This adds an error message for GSO failure, which was missing for tap. I 
> will note that in the next version.