i40e: add AF_XDP zero-copy Tx support

This patch adds zero-copy Tx support for AF_XDP sockets. It implements
the ndo_xsk_async_xmit netdev ndo and performs all the Tx logic from a
NAPI context. This means pulling egress packets from the Tx ring,
placing the frames on the NIC HW descriptor ring and completing sent
frames back to the application via the completion ring.

The regular XDP Tx ring is used for AF_XDP as well. This rationale for
this is as follows: XDP_REDIRECT guarantees mutual exclusion between
different NAPI contexts based on CPU id. In other words, a netdev can
XDP_REDIRECT to another netdev with a different NAPI context, since
the operation is bound to a specific core and each core has its own
hardware ring.

As the AF_XDP Tx action is running in the same NAPI context and using
the same ring, it will also be protected from XDP_REDIRECT actions
with the exact same mechanism.

As with AF_XDP Rx, all AF_XDP Tx specific functions are added to
i40e_xsk.c.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

drivers/net/ethernet/intel/i40e/i40e_main.c

@@ -3074,6 +3074,9 @@ static int i40e_configure_tx_ring(struct i40e_ring *ring)
 	i40e_status err = 0;
 	u32 qtx_ctl = 0;
 
+	if (ring_is_xdp(ring))
+		ring->xsk_umem = i40e_xsk_umem(ring);
+
 	/* some ATR related tx ring init */
 	if (vsi->back->flags & I40E_FLAG_FD_ATR_ENABLED) {
 		ring->atr_sample_rate = vsi->back->atr_sample_rate;
@@ -12185,6 +12188,7 @@ static const struct net_device_ops i40e_netdev_ops = {
 	.ndo_bridge_setlink	= i40e_ndo_bridge_setlink,
 	.ndo_bpf		= i40e_xdp,
 	.ndo_xdp_xmit		= i40e_xdp_xmit,
+	.ndo_xsk_async_xmit	= i40e_xsk_async_xmit,
 };
 
 /**

drivers/net/ethernet/intel/i40e/i40e_txrx.c

@@ -2597,7 +2597,11 @@ int i40e_napi_poll(struct napi_struct *napi, int budget)
 	 * budget and be more aggressive about cleaning up the Tx descriptors.
 	 */
 	i40e_for_each_ring(ring, q_vector->tx) {
-		if (!i40e_clean_tx_irq(vsi, ring, budget)) {
+		bool wd = ring->xsk_umem ?
+			  i40e_clean_xdp_tx_irq(vsi, ring, budget) :
+			  i40e_clean_tx_irq(vsi, ring, budget);
+
+		if (!wd) {
 			clean_complete = false;
 			continue;
 		}

drivers/net/ethernet/intel/i40e/i40e_xsk.c

@@ -659,3 +659,176 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int budget)
 	return failure ? budget : (int)total_rx_packets;
 }
 
+/**
+ * i40e_xmit_zc - Performs zero-copy Tx AF_XDP
+ * @xdp_ring: XDP Tx ring
+ * @budget: NAPI budget
+ *
+ * Returns true if the work is finished.
+ **/
+static bool i40e_xmit_zc(struct i40e_ring *xdp_ring, unsigned int budget)
+{
+	unsigned int total_packets = 0;
+	struct i40e_tx_buffer *tx_bi;
+	struct i40e_tx_desc *tx_desc;
+	bool work_done = true;
+	dma_addr_t dma;
+	u32 len;
+
+	while (budget-- > 0) {
+		if (!unlikely(I40E_DESC_UNUSED(xdp_ring))) {
+			xdp_ring->tx_stats.tx_busy++;
+			work_done = false;
+			break;
+		}
+
+		if (!xsk_umem_consume_tx(xdp_ring->xsk_umem, &dma, &len))
+			break;
+
+		dma_sync_single_for_device(xdp_ring->dev, dma, len,
+					   DMA_BIDIRECTIONAL);
+
+		tx_bi = &xdp_ring->tx_bi[xdp_ring->next_to_use];
+		tx_bi->bytecount = len;
+
+		tx_desc = I40E_TX_DESC(xdp_ring, xdp_ring->next_to_use);
+		tx_desc->buffer_addr = cpu_to_le64(dma);
+		tx_desc->cmd_type_offset_bsz =
+			build_ctob(I40E_TX_DESC_CMD_ICRC
+				   | I40E_TX_DESC_CMD_EOP,
+				   0, len, 0);
+		total_packets++;
+
+		xdp_ring->next_to_use++;
+		if (xdp_ring->next_to_use == xdp_ring->count)
+			xdp_ring->next_to_use = 0;
+	}
+
+	if (total_packets > 0) {
+		/* Request an interrupt for the last frame and bump tail ptr. */
+		tx_desc->cmd_type_offset_bsz |= (I40E_TX_DESC_CMD_RS <<
+						 I40E_TXD_QW1_CMD_SHIFT);
+		i40e_xdp_ring_update_tail(xdp_ring);
+
+		xsk_umem_consume_tx_done(xdp_ring->xsk_umem);
+	}
+
+	return !!budget && work_done;
+}
+
+/**
+ * i40e_clean_xdp_tx_buffer - Frees and unmaps an XDP Tx entry
+ * @tx_ring: XDP Tx ring
+ * @tx_bi: Tx buffer info to clean
+ **/
+static void i40e_clean_xdp_tx_buffer(struct i40e_ring *tx_ring,
+				     struct i40e_tx_buffer *tx_bi)
+{
+	xdp_return_frame(tx_bi->xdpf);
+	dma_unmap_single(tx_ring->dev,
+			 dma_unmap_addr(tx_bi, dma),
+			 dma_unmap_len(tx_bi, len), DMA_TO_DEVICE);
+	dma_unmap_len_set(tx_bi, len, 0);
+}
+
+/**
+ * i40e_clean_xdp_tx_irq - Completes AF_XDP entries, and cleans XDP entries
+ * @tx_ring: XDP Tx ring
+ * @tx_bi: Tx buffer info to clean
+ *
+ * Returns true if cleanup/tranmission is done.
+ **/
+bool i40e_clean_xdp_tx_irq(struct i40e_vsi *vsi,
+			   struct i40e_ring *tx_ring, int napi_budget)
+{
+	unsigned int ntc, total_bytes = 0, budget = vsi->work_limit;
+	u32 i, completed_frames, frames_ready, xsk_frames = 0;
+	struct xdp_umem *umem = tx_ring->xsk_umem;
+	u32 head_idx = i40e_get_head(tx_ring);
+	bool work_done = true, xmit_done;
+	struct i40e_tx_buffer *tx_bi;
+
+	if (head_idx < tx_ring->next_to_clean)
+		head_idx += tx_ring->count;
+	frames_ready = head_idx - tx_ring->next_to_clean;
+
+	if (frames_ready == 0) {
+		goto out_xmit;
+	} else if (frames_ready > budget) {
+		completed_frames = budget;
+		work_done = false;
+	} else {
+		completed_frames = frames_ready;
+	}
+
+	ntc = tx_ring->next_to_clean;
+
+	for (i = 0; i < completed_frames; i++) {
+		tx_bi = &tx_ring->tx_bi[ntc];
+
+		if (tx_bi->xdpf)
+			i40e_clean_xdp_tx_buffer(tx_ring, tx_bi);
+		else
+			xsk_frames++;
+
+		tx_bi->xdpf = NULL;
+		total_bytes += tx_bi->bytecount;
+
+		if (++ntc >= tx_ring->count)
+			ntc = 0;
+	}
+
+	tx_ring->next_to_clean += completed_frames;
+	if (unlikely(tx_ring->next_to_clean >= tx_ring->count))
+		tx_ring->next_to_clean -= tx_ring->count;
+
+	if (xsk_frames)
+		xsk_umem_complete_tx(umem, xsk_frames);
+
+	i40e_arm_wb(tx_ring, vsi, budget);
+	i40e_update_tx_stats(tx_ring, completed_frames, total_bytes);
+
+out_xmit:
+	xmit_done = i40e_xmit_zc(tx_ring, budget);
+
+	return work_done && xmit_done;
+}
+
+/**
+ * i40e_xsk_async_xmit - Implements the ndo_xsk_async_xmit
+ * @dev: the netdevice
+ * @queue_id: queue id to wake up
+ *
+ * Returns <0 for errors, 0 otherwise.
+ **/
+int i40e_xsk_async_xmit(struct net_device *dev, u32 queue_id)
+{
+	struct i40e_netdev_priv *np = netdev_priv(dev);
+	struct i40e_vsi *vsi = np->vsi;
+	struct i40e_ring *ring;
+
+	if (test_bit(__I40E_VSI_DOWN, vsi->state))
+		return -ENETDOWN;
+
+	if (!i40e_enabled_xdp_vsi(vsi))
+		return -ENXIO;
+
+	if (queue_id >= vsi->num_queue_pairs)
+		return -ENXIO;
+
+	if (!vsi->xdp_rings[queue_id]->xsk_umem)
+		return -ENXIO;
+
+	ring = vsi->xdp_rings[queue_id];
+
+	/* The idea here is that if NAPI is running, mark a miss, so
+	 * it will run again. If not, trigger an interrupt and
+	 * schedule the NAPI from interrupt context. If NAPI would be
+	 * scheduled here, the interrupt affinity would not be
+	 * honored.
+	 */
+	if (!napi_if_scheduled_mark_missed(&ring->q_vector->napi))
+		i40e_force_wb(vsi, ring->q_vector);
+
+	return 0;
+}

drivers/net/ethernet/intel/i40e/i40e_xsk.h

@@ -18,4 +18,8 @@ void i40e_zca_free(struct zero_copy_allocator *alloc, unsigned long handle);
 bool i40e_alloc_rx_buffers_zc(struct i40e_ring *rx_ring, u16 cleaned_count);
 int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int budget);
 
+bool i40e_clean_xdp_tx_irq(struct i40e_vsi *vsi,
+			   struct i40e_ring *tx_ring, int napi_budget);
+int i40e_xsk_async_xmit(struct net_device *dev, u32 queue_id);
+
 #endif /* _I40E_XSK_H_ */