net/mlx5e: Use fragments of the same size in non-linear legacy RQ with XDP

XDP multi buffer implementation in the kernel assumes that all fragments
have the same size. bpf_xdp_frags_increase_tail uses this assumption to
get the size of the last fragment, and __xdp_build_skb_from_frame uses
it to calculate truesize as nr_frags * xdpf->frame_sz.

The current implementation of mlx5e uses fragments of different size in
non-linear legacy RQ. Specifically, the last fragment can be larger than
the others. It's an optimization for packets smaller than MTU.

This commit adapts mlx5e to the kernel limitations and makes it use
fragments of the same size, in order to add support for XDP multi
buffer. The change is applied only if XDP is active, otherwise the old
optimization still applies.
Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

drivers/net/ethernet/mellanox/mlx5/core/en/params.c

@@ -398,8 +398,12 @@ void mlx5e_build_create_cq_param(struct mlx5e_create_cq_param *ccp, struct mlx5e
 	};
 }
 
-static int mlx5e_max_nonlinear_mtu(int first_frag_size, int frag_size)
+static int mlx5e_max_nonlinear_mtu(int first_frag_size, int frag_size, bool xdp)
 {
+	if (xdp)
+		/* XDP requires all fragments to be of the same size. */
+		return first_frag_size + (MLX5E_MAX_RX_FRAGS - 1) * frag_size;
+
 	/* Optimization for small packets: the last fragment is bigger than the others. */
 	return first_frag_size + (MLX5E_MAX_RX_FRAGS - 2) * frag_size + PAGE_SIZE;
 }
@@ -438,12 +442,14 @@ static int mlx5e_build_rq_frags_info(struct mlx5_core_dev *mdev,
 	headroom = mlx5e_get_linear_rq_headroom(params, xsk);
 	first_frag_size_max = SKB_WITH_OVERHEAD(frag_size_max - headroom);
 
-	max_mtu = mlx5e_max_nonlinear_mtu(first_frag_size_max, frag_size_max);
+	max_mtu = mlx5e_max_nonlinear_mtu(first_frag_size_max, frag_size_max,
+					  params->xdp_prog);
 	if (byte_count > max_mtu) {
 		frag_size_max = PAGE_SIZE;
 		first_frag_size_max = SKB_WITH_OVERHEAD(frag_size_max - headroom);
 
-		max_mtu = mlx5e_max_nonlinear_mtu(first_frag_size_max, frag_size_max);
+		max_mtu = mlx5e_max_nonlinear_mtu(first_frag_size_max, frag_size_max,
+						  params->xdp_prog);
 		if (byte_count > max_mtu) {
 			mlx5_core_err(mdev, "MTU %u is too big for non-linear legacy RQ (max %d)\n",
 				      params->sw_mtu, max_mtu);
@@ -463,13 +469,17 @@ static int mlx5e_build_rq_frags_info(struct mlx5_core_dev *mdev,
 		info->arr[i].frag_size = frag_size;
 		buf_size += frag_size;
 
+		if (params->xdp_prog) {
+			/* XDP multi buffer expects fragments of the same size. */
+			info->arr[i].frag_stride = frag_size_max;
+		} else {
 			if (i == 0) {
 				/* Ensure that headroom and tailroom are included. */
 				frag_size += headroom;
 				frag_size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 			}
-
 			info->arr[i].frag_stride = roundup_pow_of_two(frag_size);
+		}
 
 		i++;
 	}