Re: [xdp-hints] [PATCH bpf RFC-V2 1/5] xdp: rss hash types representation - Toke Høiland-Jørgensen

From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Jesper Dangaard Brouer <brouer@redhat.com>,
	bpf@vger.kernel.org, Stanislav Fomichev <sdf@google.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	martin.lau@kernel.org, ast@kernel.org, daniel@iogearbox.net,
	alexandr.lobakin@intel.com, larysa.zaremba@intel.com,
	xdp-hints@xdp-project.net, anthony.l.nguyen@intel.com,
	yoong.siang.song@intel.com, boon.leong.ong@intel.com,
	intel-wired-lan@lists.osuosl.org, pabeni@redhat.com,
	jesse.brandeburg@intel.com, kuba@kernel.org, edumazet@google.com,
	john.fastabend@gmail.com, hawk@kernel.org, davem@davemloft.net
Subject: Re: [xdp-hints] [PATCH bpf RFC-V2 1/5] xdp: rss hash types representation
Date: Wed, 29 Mar 2023 23:46:08 +0200	[thread overview]
Message-ID: <87355nnsdb.fsf@toke.dk> (raw)
In-Reply-To: <168010734324.3039990.16454026957159811204.stgit@firesoul>

Jesper Dangaard Brouer <brouer@redhat.com> writes:

> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 7133017bcd74..81d41df30695 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -721,12 +721,14 @@ __bpf_kfunc int bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *tim
>   * @hash: Return value pointer.
>   *
>   * Return:
> - * * Returns 0 on success or ``-errno`` on error.
> + * * Returns (positive) RSS hash **type** on success or ``-errno`` on error.

This change is going to break any BPF program that does:

if (!bpf_xdp_metadata_rx_hash(ctx, &hash))
  /* do something with hash */

so I think adding a second argument is better; that way, at least
breakage will be explicit instead of being a hidden change in semantics
(and the CO-RE style checking for kfuncs Alexei introduced should
trigger correctly).

But really, what we should do anyway is merge this during the -rc phase
to minimise any breakage :)

-Toke