Date:   Tue, 26 Sep 2017 13:21:40 -0400
From:   Andy Gospodarek <andy@...yhouse.net>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     davem@...emloft.net, alexei.starovoitov@...il.com,
        john.fastabend@...il.com, peter.waskiewicz.jr@...el.com,
        jakub.kicinski@...ronome.com, netdev@...r.kernel.org,
        mchan@...adcom.com
Subject: Re: [PATCH net-next 2/6] bpf: add meta pointer for direct access

On Mon, Sep 25, 2017 at 08:50:28PM +0200, Daniel Borkmann wrote:
> On 09/25/2017 08:10 PM, Andy Gospodarek wrote:
> [...]
> > First, thanks for this detailed description.  It was helpful to read
> > along with the patches.
> > 
> > My only concern about this area being generic is that you are now in a
> > state where any bpf program must know about all the bpf programs in the
> > receive pipeline before it can properly parse what is stored in the
> > meta-data and add it to an skb (or perform any other action).
> > Especially if each program adds it's own meta-data along the way.
> > 
> > Maybe this isn't a big concern based on the number of users of this
> > today, but it just starts to seem like a concern as there are these
> > hints being passed between layers that are challenging to track due to a
> > lack of a standard format for passing data between.
> 
> Btw, we do have similar kind of programmable scratch buffer also today
> wrt skb cb[] that you can program from tc side, the perf ring buffer,
> which doesn't have any fixed layout for the slots, or a per-cpu map
> where you can transfer data between tail calls for example, then tail
> calls themselves that need to coordinate, or simply mangling of packets
> itself if you will, but more below to your use case ...
> 
> > The main reason I bring this up is that Michael and I had discussed and
> > designed a way for drivers to communicate between each other that rx
> > resources could be freed after a tx completion on an XDP_REDIRECT
> > action.  Much like this code, it involved adding an new element to
> > struct xdp_md that could point to the important information.  Now that
> > there is a generic way to handle this, it would seem nice to be able to
> > leverage it, but I'm not sure how reliable this meta-data area would be
> > without the ability to mark it in some manner.
> > 
> > For additional background, the minimum amount of data needed in the case
> > Michael and I were discussing was really 2 words.  One to serve as a
> > pointer to an rx_ring structure and one to have a counter to the rx
> > producer entry.  This data could be acessed by the driver processing the
> > tx completions and callback to the driver that received the frame off the wire
> > to perform any needed processing.  (For those curious this would also require a
> > new callback/netdev op to act on this data stored in the XDP buffer.)
> 
> What you describe above doesn't seem to be fitting to the use-case of
> this set, meaning the area here is fully programmable out of the BPF
> program, the infrastructure you're describing is some sort of means of
> communication between drivers for the XDP_REDIRECT, and should be
> outside of the control of the BPF program to mangle.

OK, I understand that perspective.  I think saying this is really meant
as a BPF<->BPF communication channel for now is fine.

> You could probably reuse the base infra here and make a part of that
> inaccessible for the program with some sort of a fixed layout, but I
> haven't seen your code yet to be able to fully judge. Intention here
> is to allow for programmability within the BPF prog in a generic way,
> such that based on the use-case it can be populated in specific ways
> and propagated to the skb w/o having to define a fixed layout and
> bloat xdp_buff all the way to an skb while still retaining all the
> flexibility.

Some level of reuse might be proper, but I'd rather it be explicit for
my use since it's not exclusively something that will need to be used by
a BPF prog, but rather the driver.  I'll produce some patches this week
for reference.

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