On Wed, Aug 18, 2021 at 1:55 PM Jesper Dangaard Brouer
<jbrouer@xxxxxxxxxx> wrote:
>
>
> In previous discussions with AF_XDP maintainers (Magnus+Bjørn), I
> understood we have two challenges with metadata and BTF id.
>
> (1) AF_XDP doesn't know size of metadata area.
> (2) No room in xdp_desc to store the BTF-id.
>
> Below I propose new idea to solve (1) metadata size.
>
> To follow the discussion this is struct xdp_desc:
>
> /* Rx/Tx descriptor */
> struct xdp_desc {
> __u64 addr;
> __u32 len;
> __u32 options;
> };
>
> One option (that was rejected) was to store the BTF-id in 'options' and
> deduct the metadata size from BTF-id, but it was rejected as it blocks
> future usages of 'options'.
>
> The proposal by Magnus was to use a single bit in 'options' to say this
> descriptor contains metadata described via BTF info. And Bjørn proposed
> to store the BTF-id as the last member in metadata, as it would be
> accessible via minus-4 byte offset from packet start 'addr'. And again
> via BTF-id code can know the size of metadata area.
This is basically what Kishen had in his RFC.
> My idea is that we could store the metadata size in top-bits of 'len'
> member when we have set the 'options' bit for BTF-metadata.
What are the main advantages with this proposal compared to the former
one when we can get the length of the metadata section from the
BTF-id? When do we actually want to use the length of the metadata
section for something in user-space instead of just accessing the
members directly? Just trying to understand.
Thanks: Magnus
> -Jesper
>
