bpf: cpumap xdp_buff to skb conversion and allocation

This patch makes cpumap functional, by adding SKB allocation and
invoking the network stack on the dequeuing CPU.

For constructing the SKB on the remote CPU, the xdp_buff in converted
into a struct xdp_pkt, and it mapped into the top headroom of the
packet, to avoid allocating separate mem.  For now, struct xdp_pkt is
just a cpumap internal data structure, with info carried between
enqueue to dequeue.

If a driver doesn't have enough headroom it is simply dropped, with
return code -EOVERFLOW.  This will be picked up the xdp tracepoint
infrastructure, to allow users to catch this.

V2: take into account xdp->data_meta

V4:
 - Drop busypoll tricks, keeping it more simple.
 - Skip RPS and Generic-XDP-recursive-reinjection, suggested by Alexei

V5: correct RCU read protection around __netif_receive_skb_core.

V6: Setting TASK_RUNNING vs TASK_INTERRUPTIBLE based on talk with Rik van Riel
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -3260,6 +3260,7 @@ int do_xdp_generic(struct bpf_prog *xdp_prog, struct sk_buff *skb);
 int netif_rx(struct sk_buff *skb);
 int netif_rx_ni(struct sk_buff *skb);
 int netif_receive_skb(struct sk_buff *skb);
+int netif_receive_skb_core(struct sk_buff *skb);
 gro_result_t napi_gro_receive(struct napi_struct *napi, struct sk_buff *skb);
 void napi_gro_flush(struct napi_struct *napi, bool flush_old);
 struct sk_buff *napi_get_frags(struct napi_struct *napi);

kernel/bpf/cpumap.c

@@ -25,6 +25,9 @@
 #include <linux/kthread.h>
 #include <linux/capability.h>
 
+#include <linux/netdevice.h>   /* netif_receive_skb_core */
+#include <linux/etherdevice.h> /* eth_type_trans */
+
 /* General idea: XDP packets getting XDP redirected to another CPU,
  * will maximum be stored/queued for one driver ->poll() call.  It is
  * guaranteed that setting flush bit and flush operation happen on
@@ -179,6 +182,92 @@ static void cpu_map_kthread_stop(struct work_struct *work)
 	kthread_stop(rcpu->kthread);
 }
 
+/* For now, xdp_pkt is a cpumap internal data structure, with info
+ * carried between enqueue to dequeue. It is mapped into the top
+ * headroom of the packet, to avoid allocating separate mem.
+ */
+struct xdp_pkt {
+	void *data;
+	u16 len;
+	u16 headroom;
+	u16 metasize;
+	struct net_device *dev_rx;
+};
+
+/* Convert xdp_buff to xdp_pkt */
+static struct xdp_pkt *convert_to_xdp_pkt(struct xdp_buff *xdp)
+{
+	struct xdp_pkt *xdp_pkt;
+	int metasize;
+	int headroom;
+
+	/* Assure headroom is available for storing info */
+	headroom = xdp->data - xdp->data_hard_start;
+	metasize = xdp->data - xdp->data_meta;
+	metasize = metasize > 0 ? metasize : 0;
+	if ((headroom - metasize) < sizeof(*xdp_pkt))
+		return NULL;
+
+	/* Store info in top of packet */
+	xdp_pkt = xdp->data_hard_start;
+
+	xdp_pkt->data = xdp->data;
+	xdp_pkt->len  = xdp->data_end - xdp->data;
+	xdp_pkt->headroom = headroom - sizeof(*xdp_pkt);
+	xdp_pkt->metasize = metasize;
+
+	return xdp_pkt;
+}
+
+struct sk_buff *cpu_map_build_skb(struct bpf_cpu_map_entry *rcpu,
+				  struct xdp_pkt *xdp_pkt)
+{
+	unsigned int frame_size;
+	void *pkt_data_start;
+	struct sk_buff *skb;
+
+	/* build_skb need to place skb_shared_info after SKB end, and
+	 * also want to know the memory "truesize".  Thus, need to
+	 * know the memory frame size backing xdp_buff.
+	 *
+	 * XDP was designed to have PAGE_SIZE frames, but this
+	 * assumption is not longer true with ixgbe and i40e.  It
+	 * would be preferred to set frame_size to 2048 or 4096
+	 * depending on the driver.
+	 *   frame_size = 2048;
+	 *   frame_len  = frame_size - sizeof(*xdp_pkt);
+	 *
+	 * Instead, with info avail, skb_shared_info in placed after
+	 * packet len.  This, unfortunately fakes the truesize.
+	 * Another disadvantage of this approach, the skb_shared_info
+	 * is not at a fixed memory location, with mixed length
+	 * packets, which is bad for cache-line hotness.
+	 */
+	frame_size = SKB_DATA_ALIGN(xdp_pkt->len) + xdp_pkt->headroom +
+		SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+
+	pkt_data_start = xdp_pkt->data - xdp_pkt->headroom;
+	skb = build_skb(pkt_data_start, frame_size);
+	if (!skb)
+		return NULL;
+
+	skb_reserve(skb, xdp_pkt->headroom);
+	__skb_put(skb, xdp_pkt->len);
+	if (xdp_pkt->metasize)
+		skb_metadata_set(skb, xdp_pkt->metasize);
+
+	/* Essential SKB info: protocol and skb->dev */
+	skb->protocol = eth_type_trans(skb, xdp_pkt->dev_rx);
+
+	/* Optional SKB info, currently missing:
+	 * - HW checksum info		(skb->ip_summed)
+	 * - HW RX hash			(skb_set_hash)
+	 * - RX ring dev queue index	(skb_record_rx_queue)
+	 */
+
+	return skb;
+}
+
 static int cpu_map_kthread_run(void *data)
 {
 	struct bpf_cpu_map_entry *rcpu = data;
@@ -191,15 +280,45 @@ static int cpu_map_kthread_run(void *data)
 	 * kthread_stop signal until queue is empty.
 	 */
 	while (!kthread_should_stop() || !__ptr_ring_empty(rcpu->queue)) {
+		unsigned int processed = 0, drops = 0;
 		struct xdp_pkt *xdp_pkt;
 
+		/* Release CPU reschedule checks */
+		if (__ptr_ring_empty(rcpu->queue)) {
+			__set_current_state(TASK_INTERRUPTIBLE);
 			schedule();
-		/* Do work */
-		while ((xdp_pkt = ptr_ring_consume(rcpu->queue))) {
-			/* For now just "refcnt-free" */
+		} else {
+			cond_resched();
+		}
+		__set_current_state(TASK_RUNNING);
+
+		/* Process packets in rcpu->queue */
+		local_bh_disable();
+		/*
+		 * The bpf_cpu_map_entry is single consumer, with this
+		 * kthread CPU pinned. Lockless access to ptr_ring
+		 * consume side valid as no-resize allowed of queue.
+		 */
+		while ((xdp_pkt = __ptr_ring_consume(rcpu->queue))) {
+			struct sk_buff *skb;
+			int ret;
+
+			skb = cpu_map_build_skb(rcpu, xdp_pkt);
+			if (!skb) {
 				page_frag_free(xdp_pkt);
+				continue;
 			}
-		__set_current_state(TASK_INTERRUPTIBLE);
+
+			/* Inject into network stack */
+			ret = netif_receive_skb_core(skb);
+			if (ret == NET_RX_DROP)
+				drops++;
+
+			/* Limit BH-disable period */
+			if (++processed == 8)
+				break;
+		}
+		local_bh_enable(); /* resched point, may call do_softirq() */
 	}
 	__set_current_state(TASK_RUNNING);
 
@@ -490,13 +609,6 @@ static int bq_flush_to_queue(struct bpf_cpu_map_entry *rcpu,
 	return 0;
 }
 
-/* Notice: Will change in later patch */
-struct xdp_pkt {
-	void *data;
-	u16 len;
-	u16 headroom;
-};
-
 /* Runs under RCU-read-side, plus in softirq under NAPI protection.
  * Thus, safe percpu variable access.
  */
@@ -524,17 +636,13 @@ int cpu_map_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_buff *xdp,
 		    struct net_device *dev_rx)
 {
 	struct xdp_pkt *xdp_pkt;
-	int headroom;
 
-	/* For now this is just used as a void pointer to data_hard_start.
-	 * Followup patch will generalize this.
-	 */
-	xdp_pkt = xdp->data_hard_start;
+	xdp_pkt = convert_to_xdp_pkt(xdp);
+	if (!xdp_pkt)
+		return -EOVERFLOW;
 
-	/* Fake writing into xdp_pkt->data to measure overhead */
-	headroom = xdp->data - xdp->data_hard_start;
-	if (headroom < sizeof(*xdp_pkt))
-		xdp_pkt->data = xdp->data;
+	/* Info needed when constructing SKB on remote CPU */
+	xdp_pkt->dev_rx = dev_rx;
 
 	bq_enqueue(rcpu, xdp_pkt);
 	return 0;

net/core/dev.c

@@ -4492,6 +4492,33 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc)
 	return ret;
 }
 
+/**
+ *	netif_receive_skb_core - special purpose version of netif_receive_skb
+ *	@skb: buffer to process
+ *
+ *	More direct receive version of netif_receive_skb().  It should
+ *	only be used by callers that have a need to skip RPS and Generic XDP.
+ *	Caller must also take care of handling if (page_is_)pfmemalloc.
+ *
+ *	This function may only be called from softirq context and interrupts
+ *	should be enabled.
+ *
+ *	Return values (usually ignored):
+ *	NET_RX_SUCCESS: no congestion
+ *	NET_RX_DROP: packet was dropped
+ */
+int netif_receive_skb_core(struct sk_buff *skb)
+{
+	int ret;
+
+	rcu_read_lock();
+	ret = __netif_receive_skb_core(skb, false);
+	rcu_read_unlock();
+
+	return ret;
+}
+EXPORT_SYMBOL(netif_receive_skb_core);
+
 static int __netif_receive_skb(struct sk_buff *skb)
 {
 	int ret;