Re: [PATCH RFC v2 01/33] xdp: add frame size to xdp_buff

Jesper Dangaard Brouer
2020-04-14T14:16:51Z

On Wed, 8 Apr 2020 10:53:39 -0700
Jakub Kicinski <kuba@kernel.org> wrote:

> > + * This macro reserves tailroom in the XDP buffer by limiting the
> > + * XDP/BPF data access to data_hard_end. Notice same area (and size)
> > + * is used for XDP_PASS, when constructing the SKB via build_skb().
> > + */
> > +#define xdp_data_hard_end(xdp) \
> > + ((xdp)->data_hard_start + (xdp)->frame_sz - \
> > + SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))
>
> I think it should be said somewhere that the drivers are expected to
> DMA map memory up to xdp_data_hard_end(xdp).

No, I don't want driver to DMA map memory up to xdp_data_hard_end(xdp).

The driver will/should map up-to the configured MTU. Reading ahead, I
can see that you worry about XDP_TX, that doesn't do the MTU check in
xdp_ok_fwd_dev() like we do for XDP_REDIRECT. Guess, we need to check
that before doing XDP_TX, such that we don't DMA sync for_device, for
an area that does not included the original DMA-map area. I wonder if
that is a violation, if so, it is also problematic for adjust *head*.
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer