[xdp-hints] Re: [RFC bpf-next 5/5] selftests/bpf: Test rx_timestamp metadata in xskxceiver

[Alexander Lobakin <alexandr.lobakin@intel.com>]

From: Stanislav Fomichev <sdf@google.com>
Date: Fri, 28 Oct 2022 11:46:14 -0700

> On Fri, Oct 28, 2022 at 3:37 AM Jesper Dangaard Brouer
> <jbrouer@redhat.com> wrote:
> >
> >
> > On 28/10/2022 08.22, Martin KaFai Lau wrote:
> > > On 10/27/22 1:00 PM, Stanislav Fomichev wrote:
> > >> Example on how the metadata is prepared from the BPF context
> > >> and consumed by AF_XDP:
> > >>
> > >> - bpf_xdp_metadata_have_rx_timestamp to test whether it's supported;
> > >>    if not, I'm assuming verifier will remove this "if (0)" branch
> > >> - bpf_xdp_metadata_rx_timestamp returns a _copy_ of metadata;
> > >>    the program has to bpf_xdp_adjust_meta+memcpy it;
> > >>    maybe returning a pointer is better?
> > >> - af_xdp consumer grabs it from data-<expected_metadata_offset> and
> > >>    makes sure timestamp is not empty
> > >> - when loading the program, we pass BPF_F_XDP_HAS_METADATA+prog_ifindex
> > >>
> > >> Cc: Martin KaFai Lau <martin.lau@linux.dev>
> > >> Cc: Jakub Kicinski <kuba@kernel.org>
> > >> Cc: Willem de Bruijn <willemb@google.com>
> > >> Cc: Jesper Dangaard Brouer <brouer@redhat.com>
> > >> Cc: Anatoly Burakov <anatoly.burakov@intel.com>
> > >> Cc: Alexander Lobakin <alexandr.lobakin@intel.com>
> > >> Cc: Magnus Karlsson <magnus.karlsson@gmail.com>
> > >> Cc: Maryam Tahhan <mtahhan@redhat.com>
> > >> Cc: xdp-hints@xdp-project.net
> > >> Cc: netdev@vger.kernel.org
> > >> Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > >> ---
> > >>   .../testing/selftests/bpf/progs/xskxceiver.c  | 22 ++++++++++++++++++
> > >>   tools/testing/selftests/bpf/xskxceiver.c      | 23 ++++++++++++++++++-
> > >>   2 files changed, 44 insertions(+), 1 deletion(-)

[...]

> > IMHO sizeof() should come from a struct describing data_meta area see:
> >
> > https://github.com/xdp-project/bpf-examples/blob/master/AF_XDP-interaction/af_xdp_kern.c#L62
> 
> I guess I should've used pointers for the return type instead, something like:
> 
> extern __u64 *bpf_xdp_metadata_rx_timestamp(struct xdp_md *ctx) __ksym;
> 
> {
>    ...
>     __u64 *rx_timestamp = bpf_xdp_metadata_rx_timestamp(ctx);
>     if (rx_timestamp) {
>         bpf_xdp_adjust_meta(ctx, -(int)sizeof(*rx_timestamp));
>         __builtin_memcpy(data_meta, rx_timestamp, sizeof(*rx_timestamp));
>     }
> }
> 
> Does that look better?

I guess it will then be resolved to a direct store, right?
I mean, to smth like

	if (rx_timestamp)
		*(u32 *)data_meta = *rx_timestamp;

, where *rx_timestamp points directly to the Rx descriptor?

> 
> > >> +        if (ret != 0)
> > >> +            return XDP_DROP;
> > >> +
> > >> +        data = (void *)(long)ctx->data;
> > >> +        data_meta = (void *)(long)ctx->data_meta;
> > >> +
> > >> +        if (data_meta + sizeof(__u32) > data)
> > >> +            return XDP_DROP;
> > >> +
> > >> +        rx_timestamp = bpf_xdp_metadata_rx_timestamp(ctx);
> > >> +        __builtin_memcpy(data_meta, &rx_timestamp, sizeof(__u32));
> >
> > So, this approach first stores hints on some other memory location, and
> > then need to copy over information into data_meta area. That isn't good
> > from a performance perspective.
> >
> > My idea is to store it in the final data_meta destination immediately.
> 
> This approach doesn't have to store the hints in the other memory
> location. xdp_buff->priv can point to the real hw descriptor and the
> kfunc can have a bytecode that extracts the data from the hw
> descriptors. For this particular RFC, we can think that 'skb' is that
> hw descriptor for veth driver.

I really do think intermediate stores can be avoided with this
approach.
Oh, and BTW, if we plan to use a particular Hint in the BPF program
only, there's no need to place it in the metadata area at all, is
there? You only want to get it in your code, so just retrieve it to
the stack and that's it. data_meta is only for cases when you want
hints to appear in AF_XDP.

> 
> > Do notice that in my approach, the existing ethtool config setting and
> > socket options (for timestamps) still apply.  Thus, each individual
> > hardware hint are already configurable. Thus we already have a config
> > interface. I do acknowledge, that in-case a feature is disabled it still
> > takes up space in data_meta areas, but importantly it is NOT stored into
> > the area (for performance reasons).
> 
> That should be the case with this rfc as well, isn't it? Worst case
> scenario, that kfunc bytecode can explicitly check ethtool options and
> return false if it's disabled?

(to Jesper)

Once again, Ethtool idea doesn't work. Let's say you have roughly
50% of frames to be consumed by XDP, other 50% go to skb path and
the stack. In skb, I want as many HW data as possible: checksums,
hash and so on. Let's say in XDP prog I want only timestamp. What's
then? Disable everything but stamp and kill skb path? Enable
everything and kill XDP path?
Stanislav's approach allows you to request only fields you need from
the BPF prog directly, I don't see any reasons for adding one more
layer "oh no I won't give you checksum because it's disabled via
Ethtool".
Maybe I get something wrong, pls explain then :P

> 
> > >> +    }
> > >
> > > Thanks for the patches.  I took a quick look at patch 1 and 2 but
> > > haven't had a chance to look at the implementation details (eg.
> > > KF_UNROLL...etc), yet.
> > >
> >
> > Yes, thanks for the patches, even-though I don't agree with the
> > approach, at-least until my concerns/use-case can be resolved.
> > IMHO the best way to convince people is through code. So, thank you for
> > the effort.  Hopefully we can use some of these ideas and I can also
> > change/adjust my XDP-hints ideas to incorporate some of this :-)
> 
> Thank you for the feedback as well, appreciate it!
> Definitely, looking forward to a v2 from you with some more clarity on
> how those btf ids are handled by the bpf/af_xdp side!
> 
> > > Overall (with the example here) looks promising.  There is a lot of
> > > flexibility on whether the xdp prog needs any hint at all, which hint it
> > > needs, and how to store it.
> > >
> >
> > I do see the advantage that XDP prog only populates metadata it needs.
> > But how can we use/access this in __xdp_build_skb_from_frame() ?
> 
> I don't think __xdp_build_skb_from_frame is automagically solved by
> either proposal?

It's solved in my approach[0], so that __xdp_build_skb_from_frame()
are automatically get skb fields populated with the metadata if
available. But I always use a fixed generic structure, which can't
compete with your series in terms of flexibility (but solves stuff
like inter-vendor redirects and so on).
So in general I feel like there should be 2 options for metadata for
users:

1) I use one particular vendor and I always compile AF_XDP programs
   from fresh source code. I need to read/write only fields I want
   to. I'd go with kfunc or kptr here (but I don't think BPF progs
   should parse descriptor formats on their own, so your unroll NDO
   approach looks optimal for me for that case);
2) I use multiple vendors, pre-compiled AF_XDP programs or just old
   source code, I use veth and/or cpumap. So it's sorta
   back-forward-left-right-compatibility path. So here we could just
   use a fixed structure.

> For this proposal, there has to be some expected kernel metadata
> format that bpf programs will prepare and the kernel will understand?
> Think of it like xdp_hints_common from your proposal; the program will
> have to put together xdp_hints_skb into xdp metadata with the parts
> that can be populated into skb by the kernel.
> 
> For your btf ids proposal, it seems there has to be some extra kernel
> code to parse all possible driver btf_if formats and copy the
> metadata?

That's why I define a "generic" struct, so that its consumers
wouldn't have to if-else through a dozen of possible IDs :P

[...]

Great stuff from my PoV, I'd probably like to have some helpers for
writing this new NDO, so that small vendors wouldn't be afraid of
implementing it as Jakub mentioned. But still sorta optimal and
elegant for me, I'm not sure I want to post a "demo version" of my
series anymore :D
I feel like this way + one more "everything-compat-fixed" couple
would satisfy most of potential users.

Thanks,
Olek