netdev - Re: [PATCH v5 bpf-next 00/11] net: Add support for XDP in egress path

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Date:   Tue, 19 May 2020 16:21:27 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     David Ahern <dsahern@...il.com>,
        Toke Høiland-Jørgensen 
        <toke@...hat.com>, David Ahern <dsahern@...nel.org>,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        prashantbhole.linux@...il.com, john.fastabend@...il.com,
        ast@...nel.org, kafai@...com, songliubraving@...com, yhs@...com,
        andriin@...com, David Ahern <dahern@...italocean.com>,
        brouer@...hat.com, Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
Subject: Re: [PATCH v5 bpf-next 00/11] net: Add support for XDP in egress
 path

On Tue, 19 May 2020 15:31:20 +0200
Daniel Borkmann <daniel@...earbox.net> wrote:

> On 5/19/20 2:02 AM, David Ahern wrote:
> > On 5/18/20 3:06 PM, Daniel Borkmann wrote:  
> >>
> >> So given we neither call this hook on the skb path, nor XDP_TX nor
> >> AF_XDP's TX path, I was wondering also wrt the discussion with
> >> John if it makes sense to make this hook a property of the devmap
> >> _itself_, for example, to have a default BPF prog upon devmap
> >> creation or a dev-specific override that is passed on map update
> >> along with the dev. At least this would make it very clear where
> >> this is logically tied to and triggered from, and if needed (?)
> >> would provide potentially more flexibility on specifiying BPF
> >> progs to be called while also solving your use-case.  
> > 
> > You lost me on the 'property of the devmap.' The programs need to be per
> > netdevice, and devmap is an array of devices. Can you elaborate?  
> 
> I meant that the dev{map,hash} would get extended in a way where the
> __dev_map_update_elem() receives an (ifindex, BPF prog fd) tuple from
> user space and holds the program's ref as long as it is in the map slot.
> Then, upon redirect to the given device in the devmap, we'd execute the
> prog as well in order to also allow for XDP_DROP policy in there. Upon
> map update when we drop the dev from the map slot, we also release the
> reference to the associated BPF prog. What I mean to say wrt 'property
> of the devmap' is that this program is _only_ used in combination with
> redirection to devmap, so given we are not solving all the other egress
> cases for reasons mentioned, it would make sense to tie it logically to
> the devmap which would also make it clear from a user perspective _when_
> the prog is expected to run.

Yes, I agree.

I also have a use-case for 'cpumap' (cc. Lorenzo as I asked him to
work on it).  We want to run another XDP program on the CPU that
receives the xdp_frame, and then allow it to XDP redirect again.
It would make a lot of sense, to attach this XDP program via inserting
an BPF-prog-fd into the map as a value.

Notice that we would also need another expected-attach-type for this
case, as we want to allow XDP program to read xdp_md->ingress_ifindex,
but we don't have xdp_rxq_info any-longer. Thus, we need to remap that
to xdp_frame->dev_rx->ifindex (instead of rxq->dev->ifindex).

The practical use-case is the espressobin mvneta based ARM64 board,
that can only receive IRQs + RX-frames on CPU-0, but hardware have more
TX-queues that we would like to take advantage of on both CPUs.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer