Re: [[RFC xdp-hints] 00/16] XDP hints and AF_XDP support

Alexander Lobakin wrote:

From: Ederson de Souza <redacted>
Date: Mon,  2 Aug 2021 18:03:15 -0700

quoted

While there's some work going on different aspects of the XDP hints, I'd like
to present and ask for comments on this patch series.

XDP hints/metadata is a way for the driver to transmit information regarding a
specific XDP frame along with the frame. Following current discussions and
based on top of Saeed's early patches, this series provides the XDP hints with
one (or two, depending on how you view it) use case: RX/TX timestamps for the
igc driver.

Keeping with Saeed's patches, to enable XDP hints usage, one has to first
enable it with bpftool like:

  bpftool net xdp set dev <iface> md_btf on

quoted

From the driver perspective, support for XDP hints is achieved by:

 - Adding support for XDP_SETUP_MD_BTF operation, where it can register the BTF.

 - Adding support for XDP_QUERY_MD_BTF so user space can retrieve the BTF id.

 - Adding the relevant data to the metadata area of the XDP frame.

    - One of this relevant data is the BTF id of the BTF in use.

In order to make use of the BPF CO-RE mechanism, this series makes the driver
name of the struct for the XDP hints be called `xdp_hints___<driver_name>` (in
this series, as I'm using igc driver, it becomes `xdp_hints___igc`). This
should help BPF programs, as they can simply refer to the struct as `xdp_hints`.

A common issue is how to standardize the names of the fields in the BTF. Here,
a series of macros is provided on the `include/net/xdp.h`, that goes by
`XDP_GENERIC_` prefixes. In there, the `btf_id` field was added, that needs
to be strategically positioned at the end of the struct. Also added are the
`rx_timestamp` and  `tx_timestamp` fields, as I believe they're generic as
well. The macros also provide `u32` and `u64` types. Besides, I also ended
up adding a `valid_map` field. It should help whoever is using the XDP hints
to be sure of what is valid in that hints. It also makes the driver life
simple, as it just uses a single struct and validates fields as it fills
them.

The BPF sample `xdp_sample_pkts` was modified to demonstrate the usage of XDP
hints on BPF programs. It's a very simple example, but it shows some nice
things about it. For instance, instead of getting the struct somehow before,
it uses CO-RE to simply name the XDP hint field it's interested in and
read it using `BPF_CORE_READ`. (I also tried to use `bpf_core_field_exists` to
make it even more dynamic, but couldn't get to build it. I mention why in the
example.)

Also, as much of my interest lies in the user space side, the one using
AF_XDP, to support it a few additional things were done.

Firstly, a new "driver info" is provided, to be obtained via
`ioctl(SIOCETHTOOL)`: "xdp_headroom". This is how much XDP headroom is
required by the driver. While not really important for the RX path (as the
driver already applies that headroom to the XDP frame), it's
important for the TX path, as here, it's the application responsibility to
factor in the XDP headroom area. (Note that the TX timestamp is obtained from
the XDP frame of the transmitted packet, when that frame goes back to the
completion queue.)

A series of helpers was also added to libbpf to help manage this headroom
area. They go by the prefix " xsk_umem__adjust_", to adjust consumer and
producer data and metadata.

In order to read the XDP hints from the memory, another series of helpers was
added. They read the BTF from the BTF id, and create a hashmap of the offsets
and sizes of the fields, that is then used to actually retrieve the data.

I modified the "xdpsock" example to show the use of XDP hints on the AF_XDP
world, along with the proposed API.

Finally, I know that Michal and Alexandr (and probably others that I don't
know) are working in this same front. This RFC is not to race any other work,
instead I hope it can help in the discussion of the best solution for the
XDP hints – and I truly think it brings value, specifically for the AF_XDP
usages.

XDP Hints have been discussed on Netdev 0x15, and we kinda
established the optimal way for doing it. This RFC's approach
is not actual anymore.

Its great it was discussed, but you need to also summarize it
on the list. Give us the conclusion, who came to it, and why
its better then this proposal or whats wrong with this proposal.
Not everyone was in the discussion and here we
have a concrete proposal _with_ code. You can't just out of hand
throw it out based on a conference discussion.

You could just write to me and request write perms on our open
GitHub repo (which was mentioned here several times) for Hints
to do things if not together, then in one place at least.
I'll be off for two weeks since next Monday, Michal could get
you into things if you decide to join after than point
(if at all).

I'll review code thats posted on the list. Please
do the same or give us a _reason_ to skip it. It has a nice commit
message that on the face looks like a reasonable starting point
even if I have a few issues with the aproach in a couple spots.

Time to get coffee on my side.

Thanks,
John

Thanks,
Al