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

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 14 Apr 2020 16:16:26 +0200

On Wed, 8 Apr 2020 10:53:39 -0700
Jakub Kicinski <[email-masked]> 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