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 19:16 -0800, Alexander Duyck wrote:
> On Mon, Dec 7, 2020 at 3:03 PM Saeed Mahameed <saeed@kernel.org>
> wrote:
> > 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 ?
> 
> Yeah, the pointers have a few bits that are guaranteed 0 and in my
> mind reusing the lower bits from a 4 or 8 byte aligned pointer would
> make more sense then stealing the upper bits from the size of the
> frame.

Ha, i can't imagine how accessing that pointer would look like ..
is possible to define the pointer as a bit-field and just access it
normally ? or do we need to fix it up every time we need to access it ?
will gcc/static checkers complain about wrong pointer type ?