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Date:   Thu, 29 Nov 2018 13:59:30 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
To:     Toke =?iso-8859-1?Q?H=F8iland-J=F8rgensen?= <toke@toke.dk>
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
Message-ID: <20181129125929.GB6365@localhost.localdomain>
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> 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?

Regards,
Lorenzo

> 
> -Toke