Re: [PATCH RFC bpf-next 07/20] xdp: Track if metadata is supported in xdp_frame <> xdp_buff conversions

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Arthur <arthur@arthurfabre.com>
Date: Wed, 05 Mar 2025 15:32:04 +0100

> From: Arthur Fabre <afabre@cloudflare.com>
> 
> xdp_buff stores whether metadata is supported by a NIC by setting
> data_meta to be greater than data.
> 
> But xdp_frame only stores the metadata size (as metasize), so converting
> between xdp_frame and xdp_buff is lossy.
> 
> Steal an unused bit in xdp_frame to track whether metadata is supported
> or not.
> 
> This will lets us have "generic" functions for setting skb fields from
> either xdp_frame or xdp_buff from drivers.
> 
> Signed-off-by: Arthur Fabre <afabre@cloudflare.com>
> ---
>  include/net/xdp.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 58019fa299b56dbd45c104fdfa807f73af6e4fa4..84afe07d09efdb2ab0cb78b904f02cb74f9a56b6 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -116,6 +116,9 @@ static __always_inline void xdp_buff_set_frag_pfmemalloc(struct xdp_buff *xdp)
>  	xdp->flags |= XDP_FLAGS_FRAGS_PF_MEMALLOC;
>  }
>  
> +static bool xdp_data_meta_unsupported(const struct xdp_buff *xdp);
> +static void xdp_set_data_meta_invalid(struct xdp_buff *xdp);
> +
>  static __always_inline void *xdp_buff_traits(const struct xdp_buff *xdp)
>  {
>  	return xdp->data_hard_start + _XDP_FRAME_SIZE;
> @@ -270,7 +273,9 @@ struct xdp_frame {
>  	void *data;
>  	u32 len;
>  	u32 headroom;
> -	u32 metasize; /* uses lower 8-bits */
> +	u32	:23, /* unused */
> +		meta_unsupported:1,
> +		metasize:8;

See the history of this structure how we got rid of using bitfields here
and why.

...because of performance.

Even though metasize uses only 8 bits, 1-byte access is slower than
32-byte access.

I was going to write "you can use the fact that metasize is always a
multiple of 4 or that it's never > 252, for example, you could reuse LSB
as a flag indicating that meta is not supported", but first of all

Do we still have drivers which don't support metadata?
Why don't they do that? It's not HW-specific or even driver-specific.
They don't reserve headroom? Then they're invalid, at least XDP_REDIRECT
won't work.

So maybe we need to fix those drivers first, if there are any.

>  	/* Lifetime of xdp_rxq_info is limited to NAPI/enqueue time,
>  	 * while mem_type is valid on remote CPU.
>  	 */
> @@ -369,6 +374,8 @@ void xdp_convert_frame_to_buff(const struct xdp_frame *frame,
>  	xdp->data = frame->data;
>  	xdp->data_end = frame->data + frame->len;
>  	xdp->data_meta = frame->data - frame->metasize;
> +	if (frame->meta_unsupported)
> +		xdp_set_data_meta_invalid(xdp);
>  	xdp->frame_sz = frame->frame_sz;
>  	xdp->flags = frame->flags;
>  }
> @@ -396,6 +403,7 @@ int xdp_update_frame_from_buff(const struct xdp_buff *xdp,
>  	xdp_frame->len  = xdp->data_end - xdp->data;
>  	xdp_frame->headroom = headroom - sizeof(*xdp_frame);
>  	xdp_frame->metasize = metasize;
> +	xdp_frame->meta_unsupported = xdp_data_meta_unsupported(xdp);
>  	xdp_frame->frame_sz = xdp->frame_sz;
>  	xdp_frame->flags = xdp->flags;

Thanks,
Olek