Re: [PATCH net 2/6] ice: do not round up result of dbuff calculation for xsk pool

["Björn Töpel" <bjorn@kernel.org>]

Maciej Fijalkowski <maciej.fijalkowski@intel.com> writes:

> When programming dbuff 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.dbuff = DIV_ROUND_UP(3520, 128) ->
> 3520 / 128 = 27.5 -> round up results in 28
>
> dbuff programming unit is 128. If we give 128 * 28 = 3584. So HW will
> corrupt 64 bytes from tailroom. Doing plain division will floor the
> result and would not exceed reserved space at the end.
>
> 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 | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/ethernet/intel/ice/ice_base.c b/drivers/net/ethernet/intel/ice/ice_base.c
> index 1667f686ff75..930688bd3476 100644
> --- a/drivers/net/ethernet/intel/ice/ice_base.c
> +++ b/drivers/net/ethernet/intel/ice/ice_base.c
> @@ -498,8 +498,11 @@ static int ice_setup_rx_ctx(struct ice_rx_ring *ring)
>  	/* Receive Packet Data Buffer Size.
>  	 * The Packet Data Buffer Size is defined in 128 byte units.
>  	 */
> -	rlan_ctx.dbuf = DIV_ROUND_UP(ring->rx_buf_len,
> -				     BIT_ULL(ICE_RLAN_CTX_DBUF_S));
> +	if (ring->xsk_pool)
> +		rlan_ctx.dbuf = ring->rx_buf_len / BIT_ULL(ICE_RLAN_CTX_DBUF_S);
> +	else
> +		rlan_ctx.dbuf = DIV_ROUND_UP(ring->rx_buf_len,
> +					     BIT_ULL(ICE_RLAN_CTX_DBUF_S));

nit: Maybe put the BIT_ULL() in a variable (steps/unit?) and use ?: