Re: [PATCH net] veth: xdp: use head instead of hard_start

[Toshiaki Makita]

On 2020/03/31 12:56, maowenan wrote:
> On 2020/3/31 7:35, Toshiaki Makita wrote:
> > Hi Mao & Jesper
> > (Resending with plain text...)
> >
> > On 2020/03/30 20:34, Jesper Dangaard Brouer wrote:
> > > On Mon, 30 Mar 2020 18:26:31 +0800
> > > Mao Wenan <maowenan@xxxxxxxxxx> wrote:
> > >
> > > > xdp.data_hard_start is mapped to the first
> > > > address of xdp_frame, but the pointer hard_start
> > > > is the offset(sizeof(struct xdp_frame)) of xdp_frame,
> > > > it should use head instead of hard_start to
> > > > set xdp.data_hard_start. Otherwise, if BPF program
> > > > calls helper_function such as bpf_xdp_adjust_head, it
> > > > will be confused for xdp_frame_end.
> > >
> > > I have noticed this[1] and have a patch in my current patchset for
> > > fixing this. IMHO is is not so important fix right now, as the effect
> > > is that you currently only lose 32 bytes of headroom.
> I consider that it is needed because bpf_xdp_adjust_head() just a common helper function,
> veth as one driver application should keep the same as 32 bytes of headroom as other driver.
> And convert_to_xdp_frame set() also store info in top of packet, and set:
> 	xdp_frame = xdp->data_hard_start;
>
> > > [1] https://lore.kernel.org/netdev/158446621887.702578.17234304084556809684.stgit@firesoul/
> >
> > You are right, the subtraction is not necessary here.
> I guess you mean that previous subtraction is not necessary ? this line : void *head = hard_start - sizeof(struct xdp_frame); ?

No I just mean subtraction of headroom is not necessary, and I noticed this 
description was confusing. Sorry about that.
You can use "head" for data_hard_start.

Toshiaki Makita