Re: [PATCH v5 bpf-next 01/14] xdp: introduce mb in xdp_buff/xdp_frame

[Saeed Mahameed <saeed@kernel.org>]

On Mon, 2020-12-07 at 13:16 -0800, Alexander Duyck wrote:
> On Mon, Dec 7, 2020 at 8:36 AM Lorenzo Bianconi <lorenzo@kernel.org>
> wrote:
> > Introduce multi-buffer bit (mb) in xdp_frame/xdp_buffer data
> > structure
> > in order to specify if this is a linear buffer (mb = 0) or a multi-
> > buffer
> > frame (mb = 1). In the latter case the shared_info area at the end
> > of the
> > first buffer is been properly initialized to link together
> > subsequent
> > buffers.
> > 
> > Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> > ---
> >  include/net/xdp.h | 8 ++++++--
> >  net/core/xdp.c    | 1 +
> >  2 files changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/net/xdp.h b/include/net/xdp.h
> > index 700ad5db7f5d..70559720ff44 100644
> > --- a/include/net/xdp.h
> > +++ b/include/net/xdp.h
> > @@ -73,7 +73,8 @@ struct xdp_buff {
> >         void *data_hard_start;
> >         struct xdp_rxq_info *rxq;
> >         struct xdp_txq_info *txq;
> > -       u32 frame_sz; /* frame size to deduce
> > data_hard_end/reserved tailroom*/
> > +       u32 frame_sz:31; /* frame size to deduce
> > data_hard_end/reserved tailroom*/
> > +       u32 mb:1; /* xdp non-linear buffer */
> >  };
> > 
> 
> If we are really going to do something like this I say we should just
> rip a swath of bits out instead of just grabbing one. We are already
> cutting the size down then we should just decide on the minimum size
> that is acceptable and just jump to that instead of just stealing one
> bit at a time. It looks like we already have differences between the
> size here and frame_size in xdp_frame.
> 

+1

> If we have to steal a bit why not look at something like one of the
> lower 2/3 bits in rxq? You could then do the same thing using dev_rx
> in a similar fashion instead of stealing from a bit that is likely to
> be used in multiple spots and modifying like this adds extra overhead
> to?
> 

What do you mean in rxq ? from the pointer ?