Re: [PATCH v2 bpf-next 2/2] net: xdp: introduce xdp_prepare_buff utility routine

[Saeed Mahameed <saeed@kernel.org>]

On Fri, 2020-12-11 at 20:28 +0100, Lorenzo Bianconi wrote:
> Introduce xdp_prepare_buff utility routine to initialize per-
> descriptor
> xdp_buff fields (e.g. xdp_buff pointers). Rely on xdp_prepare_buff()
> in
> all XDP capable drivers.
> 
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> 
...
> +static inline void
> +xdp_prepare_buff(struct xdp_buff *xdp, unsigned char *hard_start,
> +		 int headroom, int data_len)
> +{
> +	xdp->data_hard_start = hard_start;
> +	xdp->data = hard_start + headroom;
> +	xdp->data_end = xdp->data + data_len;
> +	xdp->data_meta = xdp->data;

You might want to compute data = hard_start + headroom; on a local var,
and hopefully gcc will put it into a register, then reuse it three
times instead of the 2 xdp->data de-references you got at the end of
the function.

unsigned char *data = hard_start + headroom;

xdp->data_hard_start = hard_start;
xdp->data = data;
xdp->data_end = data + data_len;
xdp->data_meta = data;