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

[Lorenzo Bianconi <lorenzo.bianconi@redhat.com>]

> 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 :)

> 
> >> 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...
> 

Correct.

Regards,
Lorenzo

> -Toke