From: Arthur <arthur@xxxxxxxxxxxxxxx>
Date: Wed, 05 Mar 2025 15:32:04 +0100
> From: Arthur Fabre <afabre@xxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxx>
> ---
> 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
