Re: [PATCH v2 net 3/8] ice: do not round up result of dbuf calculation for xsk pool

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

On Fri, Mar 20, 2026 at 09:18:19AM +0100, Björn Töpel wrote:
> Maciej Fijalkowski <maciej.fijalkowski@intel.com> writes:
> 
> > When programming dbuf on rx queue context, avoid division round up as
> > it causes to actually corrupt the tailroom for AF_XDP ZC. Below is an
> > example based on 4k chunk size when xsk pool pointer is valid on given
> > rx ring:
> >
> > chunk_size = 4096
> > headroom = 256
> > tailroom = 320
> >
> > ring->rx_buf_len = 4096 - 256 - 320 = 3520
> >
> > rx_ctx.dbuf = DIV_ROUND_UP(3520, 128) ->
> > 3520 / 128 = 27.5 -> round up results in 28
> >
> > dbuf programming unit is 128. If we give 128 * 28 = 3584. So HW will
> > corrupt 64 bytes from tailroom. Decrement dbuf by 1 when xsk_pool is
> > present on given ice_rx_ring.
> >
> > Also, restore ::rx_buf_len setting via xsk_pool_get_rx_frame_size() as
> > of now it respects the tailroom.
> >
> > Fixes: 1bbc04de607b ("ice: xsk: add RX multi-buffer support")
> > Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> > ---
> >  drivers/net/ethernet/intel/ice/ice_base.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/drivers/net/ethernet/intel/ice/ice_base.c b/drivers/net/ethernet/intel/ice/ice_base.c
> > index 1667f686ff75..f9514d7bb83c 100644
> > --- a/drivers/net/ethernet/intel/ice/ice_base.c
> > +++ b/drivers/net/ethernet/intel/ice/ice_base.c
> > @@ -500,6 +500,8 @@ static int ice_setup_rx_ctx(struct ice_rx_ring *ring)
> >  	 */
> >  	rlan_ctx.dbuf = DIV_ROUND_UP(ring->rx_buf_len,
> >  				     BIT_ULL(ICE_RLAN_CTX_DBUF_S));
> > +	if (ring->xsk_pool)
> > +		rlan_ctx.dbuf--;
> 
> Hmm, wont this be overly pessimistic? Smth like
> 
>   if (ring->xsk_pool)
>     rlan_ctx.dbuf = ring->rx_buf_len >> ICE_RLAN_CTX_DBUF_S;
>   // else round up?

Ok!

> 
>