netdev - Re: [ovs-dev] [PATCH net-next 2/2] net: openvswitch: add drop action

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Date: Fri, 7 Jul 2023 18:04:36 +0200
From: Ilya Maximets <i.maximets@....org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: i.maximets@....org, Eric Garver <eric@...ver.life>,
 Aaron Conole <aconole@...hat.com>, netdev@...r.kernel.org,
 dev@...nvswitch.org, Paolo Abeni <pabeni@...hat.com>,
 Eric Dumazet <edumazet@...gle.com>, "David S. Miller" <davem@...emloft.net>,
 Adrian Moreno <amorenoz@...hat.com>, Eelco Chaudron <echaudro@...hat.com>
Subject: Re: [ovs-dev] [PATCH net-next 2/2] net: openvswitch: add drop action

On 7/7/23 17:29, Ilya Maximets wrote:
> On 7/7/23 17:00, Jakub Kicinski wrote:
>> On Fri, 7 Jul 2023 12:30:38 +0200 Ilya Maximets wrote:
>>> A wild idea:  How about we do not define actual reasons?  i.e. define a
>>> subsystem and just call kfree_skb_reason(skb, SUBSYSTEM | value), where
>>> 'value' is whatever userspace gives as long as it is within a subsystem
>>> range?
>>
>> That already exists, right? Johannes added it in the last release for WiFi.
> 
> I'm not sure.  The SKB_DROP_REASON_SUBSYS_MAC80211_UNUSABLE behaves similarly
> to that on a surface.  However, looking closer, any value that can be passed
> into ieee80211_rx_handlers_result() and ends up in the kfree_skb_reason() is
> kind of defined in net/mac80211/drop.h, unless I'm missing something (very
> possible, because I don't really know wifi code).
> 
> The difference, I guess, is that for openvswitch values will be provided by
> the userpsace application via netlink interface.  It'll be just a number not
> defined anywhere in the kernel.  Only the subsystem itself will be defined
> in order to occupy the range.  Garbage in, same garbage out, from the kernel's
> perspective.

To be clear, I think, not defining them in this particular case is better.
Definition of every reason that userspace can come up with will add extra
uAPI maintenance cost/issues with no practical benefits.  Values are not
going to be used for anything outside reporting a drop reason and subsystem
offset is not part of uAPI anyway.

> 
> Best regards, Ilya Maximets.