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

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Message-ID: <f4501f54-875a-4c46-9e77-802bd81f4230@daynix.com>
Date: Sat, 12 Oct 2024 19:57:22 +0900
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Jonathan Corbet <corbet@....net>, Jason Wang <jasowang@...hat.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 "Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo
 <xuanzhuo@...ux.alibaba.com>, Shuah Khan <shuah@...nel.org>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, kvm@...r.kernel.org,
 virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org,
 Yuri Benditovich <yuri.benditovich@...nix.com>,
 Andrew Melnychenko <andrew@...nix.com>,
 Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com
Subject: Re: [PATCH RFC v5 04/10] tun: Unify vnet implementation

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

> 
> 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.