Re: [RFC 0/5] add XDP support to mt76x2e/mt76x0e drivers

["Toke Høiland-Jørgensen" <toke@toke.dk>]

Lorenzo Bianconi <lorenzo.bianconi@redhat.com> writes:

>> Lorenzo Bianconi <lorenzo.bianconi@redhat.com> writes:
>> 
>> > On Nov 28, Toke Høiland-Jørgensen wrote:
>> >> Lorenzo Bianconi <lorenzo.bianconi@redhat.com> writes:
>> >> 
>> >> >> 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.
>> >> >
>> >> > Yes, here it is a little bit harder, but I was meaning that the bpf program
>> >> > has to be very careful when dropping a packet :)
>> >> 
>> >> Yeah. What kind of filtering were you thinking you would use this for in
>> >> the short term?
>> >> 
>> >
>> > When I started working on XDP for mt76 I was thinking about BSSID
>> > filtering but I was looking for a more general solution respect to add
>> > that feature in the driver. Moreover we could use bpf for fast packet
>> > filtering when you add an interface in monitor mode.
>> 
>> Yup, both of these make sense.
>> 
>> > Nevertheless I guess there could be other use cases not limited to
>> > frame filtering. My primary goal with this series is to collect
>> > ideas/concerns on WiFi XDP/eBPF possible uses cases.
>> 
>> Well, Michał's idea about offloading is cool if it is possible to get
>> vendors to implement it.
>> 
>
> Yep, would be very cool :)
>
>> Other than that, if we can solve the issues with differences between
>> 802.11 and plain Ethernet frames, I see no reason why it wouldn't be
>> possible to implement an XDP fast-path for WiFi-to-Ethernet forwarding,
>> which might be useful in an access point, especially as WiFi speeds
>> increase.
>> 
>
> Agree
>
>> The other direction will probably be more difficult, at least if 802.11
>> frames need to be built in software. It *might* be possible with the XDP
>> egress hook we are planning (with a suitable set of helpers, the eBPF
>> program could build the 802.11 frames), but I'm not really sure if that
>> is worth doing as I'm quite sure there are some hairy edge cases
>> there...
>
> The possible issue with XDP_DROP action you are referring to here is
> A-MPDU reordering on rx side, right? If so I guess the issue will be
> fixed by tid_agg_rx->reorder_timer. Are you referring to other
> possible edge cases?

I'm not sure, which is why I was being deliberately vague... ;)

Just something to be aware of when going beyond proof of concept, I
guess: here be dragons.

-Toke