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

[Saeed Mahameed <saeedm@mellanox.com>]

On Wed, 2020-04-08 at 10:53 -0700, Jakub Kicinski wrote:
> On Wed, 08 Apr 2020 13:50:39 +0200 Jesper Dangaard Brouer wrote:
> > XDP have evolved to support several frame sizes, but xdp_buff was
> > not
> > updated with this information. The frame size (frame_sz) member of
> > xdp_buff is introduced to know the real size of the memory the
> > frame is
> > delivered in.
> > 
> > When introducing this also make it clear that some tailroom is
> > reserved/required when creating SKBs using build_skb().
> > 
> > It would also have been an option to introduce a pointer to
> > data_hard_end (with reserved offset). The advantage with frame_sz
> > is
> > that (like rxq) drivers only need to setup/assign this value once
> > per
> > NAPI cycle. Due to XDP-generic (and some drivers) it's not possible
> > to
> > store frame_sz inside xdp_rxq_info, because it's varies per packet
> > as it
> > can be based/depend on packet length.
> > 
> > Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> > ---
> >  include/net/xdp.h |   17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> > 
> > diff --git a/include/net/xdp.h b/include/net/xdp.h
> > index 40c6d3398458..99f4374f6214 100644
> > --- a/include/net/xdp.h
> > +++ b/include/net/xdp.h
> > @@ -6,6 +6,8 @@
> >  #ifndef __LINUX_NET_XDP_H__
> >  #define __LINUX_NET_XDP_H__
> >  
> > +#include <linux/skbuff.h> /* skb_shared_info */
> > +
> >  /**
> >   * DOC: XDP RX-queue information
> >   *
> > @@ -70,8 +72,23 @@ struct xdp_buff {
> >  	void *data_hard_start;
> >  	unsigned long handle;
> >  	struct xdp_rxq_info *rxq;
> > +	u32 frame_sz; /* frame size to deduct data_hard_end/reserved
> > tailroom*/
> 
> Perhaps
> 
> /* length of packet buffer, starting at data_hard_start */
> 
> ?
> 
> >  };
> >  
> > +/* Reserve memory area at end-of data area.
> 
> I wouldn't say this reserves anything. It just computes the end
> pointer, no?
> 
> > + *
> > + * 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).
> 

but this works on a specific xdp buff, drivers work with mtu

and what if the driver want to have this as an option per packet .. 
i.e.: if there is enough tail room, then build_skb, otherwise
alloc new skb, copy headers, setup data frags.. etc

having such limitations on driver can be very strict, i think the
decision must remain dynamic per frame..

of-course drivers should optimize to preserve enough tail room for all
rx packets.. 

> > +
> > +/* Like skb_shinfo */
> > +#define xdp_shinfo(xdp)	((struct skb_shared_info
> > *)(xdp_data_hard_end(xdp)))
> > +// XXX: Above likely belongs in later patch
> > +
> >  struct xdp_frame {
> >  	void *data;
> >  	u16 len;
> > 
> >