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

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

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.

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.

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...

-Toke