Date: Fri, 16 Jun 2023 10:32:00 -0700
From: Stanislav Fomichev <sdf@...gle.com>
To: Magnus Karlsson <magnus.karlsson@...il.com>
Cc: Toke Høiland-Jørgensen <toke@...nel.org>, 
	bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, 
	martin.lau@...ux.dev, song@...nel.org, yhs@...com, john.fastabend@...il.com, 
	kpsingh@...nel.org, haoluo@...gle.com, jolsa@...nel.org, willemb@...gle.com, 
	dsahern@...nel.org, magnus.karlsson@...el.com, bjorn@...nel.org, 
	maciej.fijalkowski@...el.com, netdev@...r.kernel.org
Subject: Re: [RFC bpf-next 0/7] bpf: netdev TX metadata

On Fri, Jun 16, 2023 at 1:13 AM Magnus Karlsson
<magnus.karlsson@...il.com> wrote:
>
> On Fri, 16 Jun 2023 at 02:09, Stanislav Fomichev <sdf@...gle.com> wrote:
> >
> > On Mon, Jun 12, 2023 at 2:01 PM Toke Høiland-Jørgensen <toke@...nel.org> wrote:
> > >
> > > Some immediate thoughts after glancing through this:
> > >
> > > > --- Use cases ---
> > > >
> > > > The goal of this series is to add two new standard-ish places
> > > > in the transmit path:
> > > >
> > > > 1. Right before the packet is transmitted (with access to TX
> > > >    descriptors)
> > > > 2. Right after the packet is actually transmitted and we've received the
> > > >    completion (again, with access to TX completion descriptors)
> > > >
> > > > Accessing TX descriptors unlocks the following use-cases:
> > > >
> > > > - Setting device hints at TX: XDP/AF_XDP might use these new hooks to
> > > > use device offloads. The existing case implements TX timestamp.
> > > > - Observability: global per-netdev hooks can be used for tracing
> > > > the packets and exploring completion descriptors for all sorts of
> > > > device errors.
> > > >
> > > > Accessing TX descriptors also means that the hooks have to be called
> > > > from the drivers.
> > > >
> > > > The hooks are a light-weight alternative to XDP at egress and currently
> > > > don't provide any packet modification abilities. However, eventually,
> > > > can expose new kfuncs to operate on the packet (or, rather, the actual
> > > > descriptors; for performance sake).
> > >
> > > dynptr?
> > >
> > > > --- UAPI ---
> > > >
> > > > The hooks are implemented in a HID-BPF style. Meaning they don't
> > > > expose any UAPI and are implemented as tracing programs that call
> > > > a bunch of kfuncs. The attach/detach operation happen via BPF syscall
> > > > programs. The series expands device-bound infrastructure to tracing
> > > > programs.
> > >
> > > Not a fan of the "attach from BPF syscall program" thing. These are part
> > > of the XDP data path API, and I think we should expose them as proper
> > > bpf_link attachments from userspace with introspection etc. But I guess
> > > the bpf_mprog thing will give us that?
> > >
> > > > --- skb vs xdp ---
> > > >
> > > > The hooks operate on a new light-weight devtx_frame which contains:
> > > > - data
> > > > - len
> > > > - sinfo
> > > >
> > > > This should allow us to have a unified (from BPF POW) place at TX
> > > > and not be super-taxing (we need to copy 2 pointers + len to the stack
> > > > for each invocation).
> > >
> > > Not sure what I think about this one. At the very least I think we
> > > should expose xdp->data_meta as well. I'm not sure what the use case for
> > > accessing skbs is? If that *is* indeed useful, probably there will also
> > > end up being a use case for accessing the full skb?
> >
> > I spent some time looking at data_meta story on AF_XDP TX and it
> > doesn't look like it's supported (at least in a general way).
> > You obviously get some data_meta when you do XDP_TX, but if you want
> > to pass something to the bpf prog when doing TX via the AF_XDP ring,
> > it gets complicated.
>
> When we designed this some 5 - 6 years ago, we thought that there
> would be an XDP for egress action in the "nearish" future that could
> be used to interpret the metadata field in front of the packet.
> Basically, the user would load an XDP egress program that would define
> the metadata layout by the operations it would perform on the metadata
> area. But since XDP on egress has not happened, you are right, there
> is definitely something missing to be able to use metadata on Tx. Or
> could your proposed hook points be used for something like this?

Thanks for the context!
Yes, the proposal is to use these new tx hooks to read out af_xdp
metadata and apply it to the packet via a bunch of tbd kfuncs.
AF_XDP and BPF programs would have to have a contract about the
metadata layout (same as we have on rx).

> > In zerocopy mode, we can probably use XDP_UMEM_UNALIGNED_CHUNK_FLAG
> > and pass something in the headroom.
>
> This feature is mainly used to allow for multiple packets on the same
> chunk (to save space) and also to be able to have packets spanning two
> chunks. Even in aligned mode, you can start a packet at an arbitrary
> address in the chunk as long as the whole packet fits into the chunk.
> So no problem having headroom in any of the modes.

But if I put it into the headroom it will only be passed down to the
driver in zero-copy mode, right?
If I do tx_desc->addr = packet_start, no medata (that goes prior to
packet_start) gets copied into skb in the copy mode (it seems).
Or do you suggest that the interface should be tx_desc->addr =
metadata_start and the bpf program should call the equivalent of
bpf_xdp_adjust_head to consume this metadata?

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