[xdp-hints] Re: [PATCH bpf-next v3 11/12] mlx5: Support RX XDP metadata

[Jakub Kicinski <kuba@kernel.org>]

On Fri, 09 Dec 2022 15:42:37 +0100 Toke Høiland-Jørgensen wrote:
> If we expect the program to do out of band probing, we could just get
> rid of the _supported() functions entirely?
> 
> I mean, to me, the whole point of having the separate _supported()
> function for each item was to have a lower-overhead way of checking if
> the metadata item was supported. But if the overhead is not actually
> lower (because both incur a function call), why have them at all? Then
> we could just change the implementation from this:
> 
> bool mlx5e_xdp_rx_hash_supported(const struct xdp_md *ctx)
> {
> 	const struct mlx5_xdp_buff *_ctx = (void *)ctx;
> 
> 	return _ctx->xdp.rxq->dev->features & NETIF_F_RXHASH;
> }
> 
> u32 mlx5e_xdp_rx_hash(const struct xdp_md *ctx)
> {
> 	const struct mlx5_xdp_buff *_ctx = (void *)ctx;
> 
> 	return be32_to_cpu(_ctx->cqe->rss_hash_result);
> }
> 
> to this:
> 
> u32 mlx5e_xdp_rx_hash(const struct xdp_md *ctx)
> {
> 	const struct mlx5_xdp_buff *_ctx = (void *)ctx;
> 
> 	if (!(_ctx->xdp.rxq->dev->features & NETIF_F_RXHASH))
>                 return 0;
> 
> 	return be32_to_cpu(_ctx->cqe->rss_hash_result);
> }

Are there no corner cases? E.g. in case of an L2 frame you'd then
expect a hash of 0? Rather than no hash? 

If I understand we went for the _supported() thing to make inlining 
the check easier than inlining the actual read of the field.
But we're told inlining is a bit of a wait.. so isn't the motivation
for the _supported() pretty much gone? And we should we go back to
returning an error from the actual read?

Is partial inlining hard? (inline just the check and generate a full
call for the read, ending up with the same code as with _supported())