netdev - Re: xdp_redirect ifindex vs port. Was: best API for returning/setting egress port?

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Date:   Wed, 19 Apr 2017 21:58:08 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Andy Gospodarek <andy@...yhouse.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <ast@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>
Subject: Re: xdp_redirect ifindex vs port. Was: best API for
 returning/setting egress port?

On Wed, Apr 19, 2017 at 09:38:44PM -0700, John Fastabend wrote:
> On 17-04-19 07:56 PM, Alexei Starovoitov wrote:
> > On Thu, Apr 20, 2017 at 12:51:31AM +0200, Daniel Borkmann wrote:
> >>
> >> Is there a concrete reason that all the proposed future cases like sockets
> >> have to be handled within the very same XDP_REDIRECT return code? F.e. why
> >> not XDP_TX_NIC that only assumes ifindex as proposed in the patch, and future
> >> ones would get a different return code f.e. XDP_TX_SK only handling sockets
> >> when we get there implementation-wise?
> > 
> > yeah. let's keep redirect to sockets, tunnels, crypto and exotic things
> > out of this discussion.
> > XDP_REDIRECT should assume L2 raw packet is being redirected to another L2 netdev.
> > If we make it too generic it will lose performance.
> > 
> > For cls_bpf the ifindex concept is symmetric. The program can access it as
> > skb->ifindex on receive and can redirect to another ifindex via bpf_redirect() helper.
> > Since netdev is not locked, it's actually big plus, since container management
> > control plane can simply delete netns+veth and it goes away. The program can
> > have dangling ifindex (if control plane is buggy and didn't update the bpf side),
> > but it's harmless. Packets that redirect to non-existing ifindex are dropped.
> > This approach already understood and works well, so for XDP I suggest to use
> > the same approach initially before starting to reinvent the wheel.
> > struct xdp_md needs ifindex field and xdp_redirect() helper that redirects
> > to L2 netdev only. That's it. Simple and easy.
> > I think the main use cases in John's and Jesper's minds is something like
> > xdpswitch where packets are coming from VMs and from physical eths and
> > being redirected to either physical eth or to VM via upcoming vhost+xdp support.
> > This xdp_md->ifindex + xdp_redirect(to_ifindex) will solve it just fine.
> 
> hmm I must be missing something, bpf_redirect() helper should be used as a
> return statement, e.g.
> 
> 	return bpf_redirect(ifindex, flags);
> 
> Its a bit awkward to use in any other way. You would have to ensure the
> program returns TC_ACT_REDIRECT in all cases I presume. It seems incredibly
> fragile and gains nothing as far as I can tell. bpf_redirect uses per_cpu
> data and expects the core to call skb_do_redirect() to push the packet into
> the correct netdev. bpf_redirect() does not modify the skb->ifindex value,
> looking at the code now.
> 
> Are you suggesting using xdp_md to store the ifindex value instead of a per
> cpu variable in the redirect helper? 

no. i'm suggesting to store in per-cpu scratch area just like cls_bpf does.

> Do you really mean the xdp_md struct in
> the uapi headers? 

yes. since 'ifindex' needs to be part of xdp_md struct in read-only way.
Just like in cls_bpf does it.
Otherwise if we attach the same program to multiple taps it won't
know which tap the traffic arriving on and won't be able to redirect properly.

> I don't see why it needs to be in the UAPI at all. If we
> don't like per cpu variables it could be pushed as part of xdp_buff I guess.

It's not about like or dont-like per-cpu scratch area.
My main point: it works just fine for cls_bpf and i'm suggesting to do
the same for xdp_redirect, since no one ever complained about that bit of cls_bpf.

> My suggestion is we could add an ifindex to the xdp_md structure and have the
> receiving NIC driver populate the value assuming it is useful to programs. But,
> if we use cls_bpf as a model then xdp_md ifindex is more or less independent of
> the redirect helper. 

exactly. arriving ifindex is independent of xdp_redirect helper.

> In my opinion we should avoid diverging cls bpf and xdp bpf
> in subtle ways like handling of ifindex and redirect.

exactly. I'm saying the same thing.
I'm not sure which part of my proposal was so confusing.
Sorry about that.