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Date: Tue, 3 Mar 2020 10:08:12 +0100
From: Jesper Dangaard Brouer <brouer@...hat.com>
To: David Ahern <dahern@...italocean.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
davem@...emloft.net, kuba@...nel.org,
prashantbhole.linux@...il.com, jasowang@...hat.com,
toke@...hat.com, mst@...hat.com, toshiaki.makita1@...il.com,
daniel@...earbox.net, john.fastabend@...il.com, ast@...nel.org,
kafai@...com, songliubraving@...com, yhs@...com, andriin@...com,
dsahern@...il.com, brouer@...hat.com
Subject: Re: [PATCH RFC v4 bpf-next 09/11] tun: Support xdp in the Tx path
for xdp_frames
On Mon, 2 Mar 2020 21:27:08 -0700
David Ahern <dahern@...italocean.com> wrote:
> On 3/2/20 11:30 AM, Alexei Starovoitov wrote:
> > On Wed, Feb 26, 2020 at 08:20:11PM -0700, David Ahern wrote:
> >> +
> >> + act = bpf_prog_run_xdp(xdp_prog, &xdp);
> >> + switch (act) {
> >> + case XDP_TX: /* for Tx path, XDP_TX == XDP_PASS */
> >> + act = XDP_PASS;
> >> + break;
> >> + case XDP_PASS:
> >> + break;
> >> + case XDP_REDIRECT:
> >> + /* fall through */
> >> + default:
> >> + bpf_warn_invalid_xdp_action(act);
> >> + /* fall through */
> >> + case XDP_ABORTED:
> >> + trace_xdp_exception(tun->dev, xdp_prog, act);
> >> + /* fall through */
> >> + case XDP_DROP:
> >> + break;
> >> + }
> >
> > patch 8 has very similar switch. Can you share the code?
>
> Most likely; I'll take a look.
>
> >
> > I'm worried that XDP_TX is a silent alias to XDP_PASS.
> > What were the reasons to go with this approach?
>
> As I stated in the cover letter:
>
> "XDP_TX on Rx means send the packet out the device it arrived
> on; given that, XDP_Tx for the Tx path is treated as equivalent to
> XDP_PASS - ie., continue on the Tx path."
I'm not really buying this. IHMO XDP_PASS should mean continue on the
Tx path, as this is a tx/egress XDP hook. I don't see a reason to
basically "remove" the XDP_TX return code at this point.
> > imo it's less error prone and extensible to warn on XDP_TX.
> > Which will mean that both XDP_TX and XDP_REDICT are not supported
> > for egress atm.
I agree.
I more see "XDP_TX" as a mirror facility... maybe there is a use-case
for bouncing packets back in the TX/Egress hook? That is future work,
but not reason disable the option now.
> I personally don't care either way; I was going with the simplest
> concept from a user perspective.
>
> >
> > Patches 8 and 9 cover tun only. I'd like to see egress hook to be
> > implemented in at least one physical NIC. Pick any hw. Something
> > that handles real frames. Adding this hook to virtual NIC is easy,
> > but it doesn't demonstrate design trade-offs one would need to
> > think through by adding egress hook to physical nic. That's why I
> > think it's mandatory to have it as part of the patch set.
> >
> > Patch 11 exposes egress to samples/bpf. It's nice, but without
> > selftests it's no go. All new features must be exercised as part of
> > selftests/bpf.
>
> Patches that exercise the rtnetlink uapi are fairly easy to do on
> single node; anything traffic related requires multiple nodes or
> namespace level capabilities. Unless I am missing something that is
> why all current XDP tests ride on top of veth; veth changes are not
> part of this set.
>
> So to be clear you are saying that all new XDP features require
> patches to a h/w nic, veth and whatever the author really cares about
> before new features like this go in?
I would say yes. XDP is founded for physical HW NICs, and we need to
show it makes sense for physical HW NICs.
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
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