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

Jesper Dangaard Brouer
2020-04-16T13:03:00Z

On Thu, 9 Apr 2020 00:50:02 +0000
Saeed Mahameed <saeedm@mellanox.com> 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 <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 */
> > +
>
> 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