[Intel-wired-lan] [PATCH intel-net 3/5] ice: xsk: do not clear status_error0 for ntu + nb_buffs descriptor

[Maciej Fijalkowski]

On Fri, Dec 10, 2021 at 11:37:46PM +0100, Alexander Lobakin wrote:
> From: Maciej Fijalkowski <maciej.fijalkowski at intel.com>
> Date: Fri, 10 Dec 2021 15:59:39 +0100
> 
> > The descriptor that ntu is pointing at when we exit
> > ice_alloc_rx_bufs_zc() should not have its corresponding DD bit cleared
> > as descriptor is not allocated in there and it is not valid for HW
> > usage.
> > 
> > The allocation routine at the entry will fill the descriptor that ntu
> > points to after it was set to ntu + nb_buffs on previous call.
> > 
> > Even the spec says:
> > "The tail pointer should be set to one descriptor beyond the last empty
> > descriptor in host descriptor ring."
> > 
> > Therefore, step away from clearing the status_error0 on ntu + nb_buffs
> > descriptor.
> > 
> > Fixes: db804cfc21e9 ("ice: Use the xsk batched rx allocation interface")
> > Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski at intel.com>
> > ---
> >  drivers/net/ethernet/intel/ice/ice_xsk.c | 7 +------
> >  1 file changed, 1 insertion(+), 6 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/intel/ice/ice_xsk.c b/drivers/net/ethernet/intel/ice/ice_xsk.c
> > index 5cb61955c1f3..874fce9fa1c3 100644
> > --- a/drivers/net/ethernet/intel/ice/ice_xsk.c
> > +++ b/drivers/net/ethernet/intel/ice/ice_xsk.c
> > @@ -394,14 +394,9 @@ bool ice_alloc_rx_bufs_zc(struct ice_rx_ring *rx_ring, u16 count)
> >  	}
> >  
> >  	ntu += nb_buffs;
> > -	if (ntu == rx_ring->count) {
> > -		rx_desc = ICE_RX_DESC(rx_ring, 0);
> > -		xdp = rx_ring->xdp_buf;
> > +	if (ntu == rx_ring->count)
> 
> Maybe use unlikely() here while at it? 1/512 (depending on ring
> size) chance is low enough.

This would make sense to me if we would have this check inside some loop
going over the buffers that we received from xsk pool.

I tried such approach probably on Tx side and Magnus said that unlikely
will move this code to the cold section at the end of the text section.

> 
> >  		ntu = 0;
> > -	}
> >  
> > -	/* clear the status bits for the next_to_use descriptor */
> > -	rx_desc->wb.status_error0 = 0;
> >  	ice_release_rx_desc(rx_ring, ntu);
> 
> This interferes with my patch in next-queue ([0]) (well, supersedes
> it to be precise).
> Tony, what would be better to do with it, just drop mine or correct
> this one (it would become an oneliner removing status_error0
> assignment then)?

Oops, sorry. This set should go to net though, not net-next, but I can
base it on top of your patch.

> 
> >  
> >  	return count == nb_buffs;
> > -- 
> > 2.33.1
> 
> [0] https://lore.kernel.org/netdev/20211130183649.1166842-2-alexandr.lobakin@intel.com
> 
> Al