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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@toke.dk>
To:     Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Cc:     Kalle Valo <kvalo@codeaurora.org>, linux-wireless@vger.kernel.org,
        nbd@nbd.name, brouer@redhat.com
Subject: Re: [RFC 0/5] add XDP support to mt76x2e/mt76x0e drivers
In-Reply-To: <20181128131141.GD2298@localhost.localdomain>
References: <cover.1543343124.git.lorenzo.bianconi@redhat.com> <8736rla4ow.fsf@purkki.adurom.net> <20181128104436.GA2298@localhost.localdomain> <87bm69v0ol.fsf@toke.dk> <20181128131141.GD2298@localhost.localdomain>
Date:   Wed, 28 Nov 2018 15:21:37 +0100
Message-ID: <875zwhuvta.fsf@toke.dk>
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Lorenzo Bianconi <lorenzo.bianconi@redhat.com> writes:

>> >> > This series is intended as a playground to start experimenting/developing
>> >> > with XDP/eBPF over WiFi and collect ideas/concerns about it.
>> >> > Introduce XDP support to mt76x2e/mt76x0e drivers. Currently supported
>> >> > actions are:
>> >> > - XDP_PASS
>> >> > - XDP_ABORTED
>> >> > - XDP_DROP
>> >> > Introduce ndo_bpf mac80211 callback in order to to load a bpf
>> >> > program into low level driver XDP rx hook.
>> >> > This series has been tested through a simple bpf program (available here:
>> >> > https://github.com/LorenzoBianconi/bpf-workspace/tree/master/mt76_xdp_stats)
>> >> > used to count frame types received by the device.
>> >> > Possible eBPF use cases could be:
>> >> > - implement new statistics through bpf maps
>> >> > - implement fast packet filtering (e.g in monitor mode)
>> >> > - ...
>> >
>> > Hi Kalle,
>> >
>> >> 
>> >> This is most likely a stupid question, but why do this in the driver and
>> >> not in mac80211 so that all drivers could benefit from it? I guess there
>> >> are reasons for that, I just can't figure that out.
>> 
>> XDP achieves its speedup by running the eBPF program inside the driver
>> NAPI loop, before the kernel even touches the data in any other capacity
>> (and in particular, before it allocates an SKB). Which kinda means the
>> hook needs to be in the driver... Could be a fallback in mac80211,
>> though; although we'd have to figure out how that interacts with Generic
>> XDP.
>> 
>> > This is an early stage implementation, at this point I would collect
>> > other people opinions/concerns about using bpf/xdp directly on 802.11
>> > frames.
>> 
>> Thanks for looking into this!
>
> Hi Toke,
>
>> 
>> I have two concerns with running XDP on 802.11 frames:
>> 
>> 1. It makes it more difficult to add other XDP actions (such as
>>    REDIRECT), as the XDP program would then have to make sure that the
>>    outer packet headers are removed before, say, redirecting the packet
>>    out of an ethernet interface. Also, if we do add redirect, we would
>>    be bypassing mac80211 entirely; to what extent would that mess up
>>    internal state?
>> 
>
> You are right, my assumption here is the logic/complexity is moved to
> the bpf program that needs to take care of all possible issues that
> can be introduced. More or less it is the same if a bpf program mess
> up with TCP segments on a wired connection, isn't it?

No, I guess not; except here it potentially applies to all packets
(things like BAW tracking), and it is *in addition* to TCP.

>> 2. UI consistency; suddenly, the user needs to know which kind of
>>    frames to expect, and XDP program reuse becomes more difficult. This
>>    may be unavoidable given the nature of XDP, but some thought needs to
>>    go into this. Especially since we wouldn't necessarily be consistent
>>    between WiFi drivers (there are fullmac devices that remove 802.11
>>    headers before sending up the frame, right?).
>> 
>
> Right, maybe can we have some kind of 'wifi' bpf helpers?

Yeah, I guess we would at least need helpers to update any internal
state in mac80211 (such as BAW), or BPF programs wouldn't even be able
to drop packets without messing things up...

-Toke