netdev - Re: [PATCH v3 net-next RFC] Generic XDP

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Date:   Fri, 14 Apr 2017 15:30:32 -0700
From:   Jakub Kicinski <kubakici@...pl>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        xdp-newbies@...r.kernel.org
Subject: Re: [PATCH v3 net-next RFC] Generic XDP

On Fri, 14 Apr 2017 12:28:15 -0700, Alexei Starovoitov wrote:
> On Fri, Apr 14, 2017 at 11:05:25AM +0200, Jesper Dangaard Brouer wrote:
> > > 
> > > We are consistently finding that there is this real need to
> > > communicate XDP capabilities, or somehow verify that the needs
> > > of an XDP program can be satisfied by a given implementation.  
> > 
> > I fully agree that we need some way to express capabilities[1]
> >   
> > > Maximum headroom is just one.  
> 
> I don't like the idea of asking program author to explain capabilities
> to the kernel. Right now the verifier already understands more about
> the program than human does. If the verifier cannot deduct from the
> insns what program will be doing, it's pretty much guarantee
> that program author has no idea either.
> If we add 'required_headroom' as an extra flag to BPF_PROG_LOAD,
> the users will just pass something like 64 or 128 whereas the program
> might only be doing IPIP encap and that will cause kernel to
> provide extra headroom for no good reason or reject the program
> whereas it could have run just fine.
> 
> So I very much agree with this part:
> > ... or somehow verify that the needs
> > of an XDP program can be satisfied by a given implementation.  
> 
> we already have cb_access, dst_needed, xdp_adjust_head flags
> that verifier discovers in the program.
> For headroom we need one more. The verifier can see
> the constant passed into bpf_xdp_adjust_head().
> It's trickier if it's a variable, but the verifier can estimate min/max
> of the variable already and worst case it can say that it
> will be XDP_PACKET_HEADROOM.
> If the program is doing variable length bpf_xdp_adjust_head(),
> the human has no idea how much they need and cannot tell kernel anyway.

If I understand correctly that the current proposal is to:

1. Assume program needs 256 (XDP_PACKET_HEADROOM) of prepend.
2. Make verifier try to lower that through byte code analysis.
3. Make the load fail if (prog->max_prepend > driver->xdp_max_prepend).

Would that not cause unnecessary load failures?  I'm worried it would
force users to do things like this:

struct map_value *val;

if (val->prep_len > 64) /* Prove to verifier this is short */
	return XDP_DROP;
if (xdp_adjust_head(xdp, -val->prep_len))
	...

Or worse not do that, use Qlogic, Broadcom, Mellanox or Netronome cards
and then be in for a nasty surprise when they try to load the same
program for Intel parts :(

I agree that trying to express capabilities to user space is quickly
going to become more confusing than helpful.  However, I think here we
need the opposite - we need an opt-in way of program requesting
guarantees about the datapath.

We may expect most users won't care and we may want to still depend on
the verifier to analyze the program and enable optimizations, but
depending on the verifier for proving prepend lengths is scary.  Or are
we just discussion the optimizations here and not the guarantees about
headroom availability?