[xdp-hints] Re: [PATCH bpf-next V1] xdp: bpf_xdp_metadata use NODEV for no device support

[Larysa Zaremba <larysa.zaremba@intel.com>]

On Wed, Feb 15, 2023 at 11:09:36AM +0100, Jesper Dangaard Brouer wrote:
> With our XDP-hints kfunc approach, where individual drivers overload the
> default implementation, it can be hard for API users to determine
> whether or not the current device driver have this kfunc available.
> 
> Change the default implementations to use an errno (ENODEV), that
> drivers shouldn't return, to make it possible for BPF runtime to
> determine if bpf kfunc for xdp metadata isn't implemented by driver.

I think it diverts ENODEV usage from its original purpose too much. Maybe 
providing information in dmesg would be a better solution?

> 
> This is intended to ease supporting and troubleshooting setups. E.g.
> when users on mailing list report -19 (ENODEV) as an error, then we can
> immediately tell them their kernel is too old.

Do you mean driver being too old, not kernel?

> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  net/core/xdp.c |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 26483935b7a4..7bb5984ae4f7 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c
> @@ -722,10 +722,12 @@ __diag_ignore_all("-Wmissing-prototypes",
>   * @timestamp: Return value pointer.
>   *
>   * Returns 0 on success or ``-errno`` on error.
> + *
> + *  -ENODEV (19): means device driver doesn't implement kfunc
>   */
>  __bpf_kfunc int bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *timestamp)
>  {
> -	return -EOPNOTSUPP;
> +	return -ENODEV;
>  }
>  
>  /**
> @@ -734,10 +736,12 @@ __bpf_kfunc int bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *tim
>   * @hash: Return value pointer.
>   *
>   * Returns 0 on success or ``-errno`` on error.
> + *
> + *  -ENODEV (19): means device driver doesn't implement kfunc
>   */
>  __bpf_kfunc int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, u32 *hash)
>  {
> -	return -EOPNOTSUPP;
> +	return -ENODEV;
>  }
>  
>  __diag_pop();

Documentation contains the following lines:

  Not all kfuncs have to be implemented by the device driver; when not
  implemented, the default ones that return ``-EOPNOTSUPP`` will be used.

If you decide to proceed with current implementation, you'd need to update them 
in v2.

> 
>