[xdp-hints] Re: [RFC bpf-next 05/23] ice: Introduce ice_xdp_buff

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Mon, Sep 04, 2023 at 08:11:09PM +0200, Larysa Zaremba wrote:
> On Mon, Sep 04, 2023 at 05:32:14PM +0200, Maciej Fijalkowski wrote:
> > On Thu, Aug 24, 2023 at 09:26:44PM +0200, Larysa Zaremba wrote:
> > > In order to use XDP hints via kfuncs we need to put
> > > RX descriptor and ring pointers just next to xdp_buff.
> > > Same as in hints implementations in other drivers, we achieve
> > > this through putting xdp_buff into a child structure.
> > 
> > Don't you mean a parent struct? xdp_buff will be 'child' of ice_xdp_buff
> > if i'm reading this right.
> >
> 
> ice_xdp_buff is a child in terms of inheritance (pointer to ice_xdp_buff could 
> replace pointer to xdp_buff, but not in reverse).
> 
> > > 
> > > Currently, xdp_buff is stored in the ring structure,
> > > so replace it with union that includes child structure.
> > > This way enough memory is available while existing XDP code
> > > remains isolated from hints.
> > > 
> > > Minimum size of the new child structure (ice_xdp_buff) is exactly
> > > 64 bytes (single cache line). To place it at the start of a cache line,
> > > move 'next' field from CL1 to CL3, as it isn't used often. This still
> > > leaves 128 bits available in CL3 for packet context extensions.
> > 
> > I believe ice_xdp_buff will be beefed up in later patches, so what is the
> > point of moving 'next' ? We won't be able to keep ice_xdp_buff in a single
> > CL anyway.
> >
> 
> It is to at least keep xdp_buff and descriptor pointer (used for every hint) in 
> a single CL, other fields are situational.

Right, something must be moved...still, would be good to see perf
before/after :)

> 
> > > 
> > > Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
> > > ---
> > >  drivers/net/ethernet/intel/ice/ice_txrx.c     |  7 +++--
> > >  drivers/net/ethernet/intel/ice/ice_txrx.h     | 26 ++++++++++++++++---
> > >  drivers/net/ethernet/intel/ice/ice_txrx_lib.h | 10 +++++++
> > >  3 files changed, 38 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/drivers/net/ethernet/intel/ice/ice_txrx.c b/drivers/net/ethernet/intel/ice/ice_txrx.c
> > > index 40f2f6dabb81..4e6546d9cf85 100644
> > > --- a/drivers/net/ethernet/intel/ice/ice_txrx.c
> > > +++ b/drivers/net/ethernet/intel/ice/ice_txrx.c
> > > @@ -557,13 +557,14 @@ ice_rx_frame_truesize(struct ice_rx_ring *rx_ring, const unsigned int size)
> > >   * @xdp_prog: XDP program to run
> > >   * @xdp_ring: ring to be used for XDP_TX action
> > >   * @rx_buf: Rx buffer to store the XDP action
> > > + * @eop_desc: Last descriptor in packet to read metadata from
> > >   *
> > >   * Returns any of ICE_XDP_{PASS, CONSUMED, TX, REDIR}
> > >   */
> > >  static void
> > >  ice_run_xdp(struct ice_rx_ring *rx_ring, struct xdp_buff *xdp,
> > >  	    struct bpf_prog *xdp_prog, struct ice_tx_ring *xdp_ring,
> > > -	    struct ice_rx_buf *rx_buf)
> > > +	    struct ice_rx_buf *rx_buf, union ice_32b_rx_flex_desc *eop_desc)
> > >  {
> > >  	unsigned int ret = ICE_XDP_PASS;
> > >  	u32 act;
> > > @@ -571,6 +572,8 @@ ice_run_xdp(struct ice_rx_ring *rx_ring, struct xdp_buff *xdp,
> > >  	if (!xdp_prog)
> > >  		goto exit;
> > >  
> > > +	ice_xdp_meta_set_desc(xdp, eop_desc);
> > 
> > I am currently not sure if for multi-buffer case HW repeats all the
> > necessary info within each descriptor for every frag? IOW shouldn't you be
> > using the ice_rx_ring::first_desc?
> > 
> > Would be good to test hints for mbuf case for sure.
> >
> 
> In the skb path, we take metadata from the last descriptor only, so this should 
> be fine. Really worth testing with mbuf though.

Ok, thanks!