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

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 16 Apr 2020 15:02:38 +0200

On Thu, 9 Apr 2020 00:50:02 +0000
Saeed Mahameed <[email-masked]> wrote:

> On Wed, 2020-04-08 at 13:50 +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 <[email-masked]>
> > ---
> >  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 */
> > +  
> 
> I think it is wrong to make xdp.h depend on skbuff.h
> we must keep xdp.h minimal and independent,

I agree, that it seems strange to have xdp.h include skbuff.h, and I'm
not happy with that approach myself, but the alternatives all looked
kind of ugly.

> the new macros should be defined in skbuff.h 

Moving #define xdp_data_hard_end(xdp) into skbuff.h also seems strange.

> >  /**
> >   * 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*/  
> 
> why u32 ? u16 should be more than enough.. 

Nope.  It need to be able to store PAGE_SIZE == 65536.

$ echo $((1<<12))
4096
$ echo $((1<<16))
65536

$ printf "0x%X\n" 65536
0x10000

> >  };
> >  
> > +/* Reserve memory area at end-of data area.
> > + *
> > + * 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)))
> > +  
> 
> this macro is not safe when unary operators are being used

The parentheses round (xdp) does make xdp_data_hard_end(&xdp) work
correctly. What other cases are you worried about?

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer