Merge branch 'xsk-multi-buffer-support'

Maciej Fijalkowski says:

====================
xsk: multi-buffer support

v6->v7:
- rebase...[Alexei]

v5->v6:
- update bpf_xdp_query_opts__last_field in patch 10 [Alexei]

v4->v5:
- align options argument size to match options from xdp_desc [Benjamin]
- cleanup skb from xdp_sock on socket termination [Toke]
- introduce new netlink attribute for letting user space know about Tx
  frag limit; this substitutes xdp_features flag previously dedicated
  for setting ZC multi-buffer support [Toke, Jakub]
- include i40e ZC multi-buffer support
- enable TOO_MANY_FRAGS for ZC on xskxceiver; this is now possible due
  to netlink attribute mentioned two bullets above

v3->v4:
-rely on ynl for adding new xdp_features flag [Jakub]
- move xskb_list to xsk_buff_pool

v2->v3:
- Fix issue with the next valid packet getting dropped after an invalid
  packet with MAX_SKB_FRAGS + 1 frags [Magnus]
- query NETDEV_XDP_ACT_ZC_SG flag within xskxceiver and act on it
- remove redundant include in xsk.c [kernel test robot]
- s/NETDEV_XDP_ACT_NDO_ZC_SG/NETDEV_XDP_ACT_ZC_SG + kernel doc [Magnus,
  Simon]

v1->v2:
- fix spelling issues in commit messages [Simon]
- remove XSK_DESC_MAX_FRAGS, use MAX_SKB_FRAGS instead [Stan, Alexei]
- add documentation patch
- fix build error from kernel test robot on patch 10

This series of patches add multi-buffer support for AF_XDP. XDP and
various NIC drivers already have support for multi-buffer packets. With
this patch set, programs using AF_XDP sockets can now also receive and
transmit multi-buffer packets both in copy as well as zero-copy mode.
ZC multi-buffer implementation is based on ice driver.

Some definitions to put us all on the same page:

* A packet consists of one or more frames

* A descriptor in one of the AF_XDP rings always refers to a single
  frame. In the case the packet consists of a single frame, the
  descriptor refers to the whole packet.

To represent a packet consisting of multiple frames, we introduce a
new flag called XDP_PKT_CONTD in the options field of the Rx and Tx
descriptors. If it is true (1) the packet continues with the next
descriptor and if it is false (0) it means this is the last descriptor
of the packet. Why the reverse logic of end-of-packet (eop) flag found
in many NICs? Just to preserve compatibility with non-multi-buffer
applications that have this bit set to false for all packets on Rx, and
the apps set the options field to zero for Tx, as anything else will
be treated as an invalid descriptor.

These are the semantics for producing packets onto XSK Tx ring
consisting of multiple frames:

* When an invalid descriptor is found, all the other
  descriptors/frames of this packet are marked as invalid and not
  completed. The next descriptor is treated as the start of a new
  packet, even if this was not the intent (because we cannot guess
  the intent). As before, if your program is producing invalid
  descriptors you have a bug that must be fixed.

* Zero length descriptors are treated as invalid descriptors.

* For copy mode, the maximum supported number of frames in a packet is
  equal to CONFIG_MAX_SKB_FRAGS + 1. If it is exceeded, all
  descriptors accumulated so far are dropped and treated as
  invalid. To produce an application that will work on any system
  regardless of this config setting, limit the number of frags to 18,
  as the minimum value of the config is 17.

* For zero-copy mode, the limit is up to what the NIC HW
  supports. User space can discover this via newly introduced
  NETDEV_A_DEV_XDP_ZC_MAX_SEGS netlink attribute.

Here is an example Tx path pseudo-code (using libxdp interfaces for
simplicity) ignoring that the umem is finite in size, and that we
eventually will run out of packets to send. Also assumes pkts.addr
points to a valid location in the umem.

void tx_packets(struct xsk_socket_info *xsk, struct pkt *pkts,
                int batch_size)
{
	u32 idx, i, pkt_nb = 0;

	xsk_ring_prod__reserve(&xsk->tx, batch_size, &idx);

	for (i = 0; i < batch_size;) {
		u64 addr = pkts[pkt_nb].addr;
		u32 len = pkts[pkt_nb].size;

		do {
			struct xdp_desc *tx_desc;

			tx_desc = xsk_ring_prod__tx_desc(&xsk->tx, idx + i++);
			tx_desc->addr = addr;

			if (len > xsk_frame_size) {
				tx_desc->len = xsk_frame_size;
				tx_desc->options |= XDP_PKT_CONTD;
			} else {
				tx_desc->len = len;
				tx_desc->options = 0;
				pkt_nb++;
			}
			len -= tx_desc->len;
			addr += xsk_frame_size;

			if (i == batch_size) {
				/* Remember len, addr, pkt_nb for next
				 * iteration. Skipped for simplicity.
				 */
				break;
			}
		} while (len);
	}

	xsk_ring_prod__submit(&xsk->tx, i);
}

On the Rx path in copy mode, the xsk core copies the XDP data into
multiple descriptors, if needed, and sets the XDP_PKT_CONTD flag as
detailed before. Zero-copy mode in order to avoid the copies has to
maintain a chain of xdp_buff_xsk structs that represent whole packet.
This is because what actually is redirected is the xdp_buff and we
currently have no equivalent mechanism that is used for copy mode
(embedded skb_shared_info in xdp_buff) to carry the frags. This means
xdp_buff_xsk grows in size but these members are at the end and should
not be touched when data path is not dealing with fragmented packets.
This solution kept us within assumed performance impact, hence we
decided to proceed with it.

When the application gets a descriptor with the
XDP_PKT_CONTD flag set to one, it means that the packet consists of
multiple buffers and it continues with the next buffer in the following
descriptor. When a descriptor with XDP_PKT_CONTD == 0 is received, it
means that this is the last buffer of the packet. AF_XDP guarantees that
only a complete packet (all frames in the packet) is sent to the
application.

If application reads a batch of descriptors, using for example the libxdp
interfaces, it is not guaranteed that the batch will end with a full
packet. It might end in the middle of a packet and the rest of the
buffers of that packet will arrive at the beginning of the next batch,
since the libxdp interface does not read the whole ring (unless you
have an enormous batch size or a very small ring size).

Here is a simple Rx path pseudo-code example (using libxdp interfaces for
simplicity). Error paths have been excluded for simplicity:

void rx_packets(struct xsk_socket_info *xsk)
{
	static bool new_packet = true;
	u32 idx_rx = 0, idx_fq = 0;
	static char *pkt;

	int rcvd = xsk_ring_cons__peek(&xsk->rx, opt_batch_size, &idx_rx);

	xsk_ring_prod__reserve(&xsk->umem->fq, rcvd, &idx_fq);

	for (int i = 0; i < rcvd; i++) {
		struct xdp_desc *desc = xsk_ring_cons__rx_desc(&xsk->rx, idx_rx++);
		char *frag = xsk_umem__get_data(xsk->umem->buffer, desc->addr);
		bool eop = !(desc->options & XDP_PKT_CONTD);

		if (new_packet)
			pkt = frag;
		else
			add_frag_to_pkt(pkt, frag);

		if (eop)
			process_pkt(pkt);

		new_packet = eop;

		*xsk_ring_prod__fill_addr(&xsk->umem->fq, idx_fq++) = desc->addr;
	}

	xsk_ring_prod__submit(&xsk->umem->fq, rcvd);
	xsk_ring_cons__release(&xsk->rx, rcvd);
}

We had to introduce a new bind flag (XDP_USE_SG) on the AF_XDP level to
enable multi-buffer support. The reason we need to differentiate between
non multi-buffer and multi-buffer is the behaviour when the kernel gets
a packet that is larger than the frame size. Without multi-buffer, this
packet is dropped and marked in the stats. With multi-buffer on, we want
to split it up into multiple frames instead.

At the start, we thought that riding on the .frags section name of
the XDP program was a good idea. You do not have to introduce yet
another flag and all AF_XDP users must load an XDP program anyway
to get any traffic up to the socket, so why not just say that the XDP
program decides if the AF_XDP socket should get multi-buffer packets
or not? The problem is that we can create an AF_XDP socket that is Tx
only and that works without having to load an XDP program at
all. Another problem is that the XDP program might change during the
execution, so we would have to check this for every single packet.

Here is the observed throughput when compared to a codebase without any
multi-buffer changes and measured with xdpsock for 64B packets.
Apparently ZC Tx takes a hit from explicit zero length descriptors
validation. Overall, in terms of ZC performance, there is a room for
improvement, but for now we think this work is in a good shape in terms
of correctness and functionality. We were targetting for up to 5%
overhead though. Note that ZC performance drops come from core + driver
support being combined, whereas copy mode had already driver support in
place.

Mode     rxdrop       l2fwd       txonly
ice-zc    -4%          -7%         -6%
i40e-zc   -7%          -6%         -7%
drv       -1.2%         0%         +2%
skb       -0.6%        -1%         +2%

Thank you,
Tirthendu, Magnus and Maciej
====================

Link: https://lore.kernel.org/r/20230719132421.584801-1-maciej.fijalkowski@intel.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Alexei Starovoitov

Documentation/netlink/specs/netdev.yaml

@@ -62,6 +62,12 @@ attribute-sets:
         type: u64
         enum: xdp-act
         enum-as-flags: true
+      -
+        name: xdp_zc_max_segs
+        doc: max fragment count supported by ZC driver
+        type: u32
+        checks:
+          min: 1
 
 operations:
   list:

Documentation/networking/af_xdp.rst

@@ -462,8 +462,92 @@ XDP_OPTIONS getsockopt
 Gets options from an XDP socket. The only one supported so far is
 XDP_OPTIONS_ZEROCOPY which tells you if zero-copy is on or not.
 
+Multi-Buffer Support
+====================
+
+With multi-buffer support, programs using AF_XDP sockets can receive
+and transmit packets consisting of multiple buffers both in copy and
+zero-copy mode. For example, a packet can consist of two
+frames/buffers, one with the header and the other one with the data,
+or a 9K Ethernet jumbo frame can be constructed by chaining together
+three 4K frames.
+
+Some definitions:
+
+* A packet consists of one or more frames
+
+* A descriptor in one of the AF_XDP rings always refers to a single
+  frame. In the case the packet consists of a single frame, the
+  descriptor refers to the whole packet.
+
+To enable multi-buffer support for an AF_XDP socket, use the new bind
+flag XDP_USE_SG. If this is not provided, all multi-buffer packets
+will be dropped just as before. Note that the XDP program loaded also
+needs to be in multi-buffer mode. This can be accomplished by using
+"xdp.frags" as the section name of the XDP program used.
+
+To represent a packet consisting of multiple frames, a new flag called
+XDP_PKT_CONTD is introduced in the options field of the Rx and Tx
+descriptors. If it is true (1) the packet continues with the next
+descriptor and if it is false (0) it means this is the last descriptor
+of the packet. Why the reverse logic of end-of-packet (eop) flag found
+in many NICs? Just to preserve compatibility with non-multi-buffer
+applications that have this bit set to false for all packets on Rx,
+and the apps set the options field to zero for Tx, as anything else
+will be treated as an invalid descriptor.
+
+These are the semantics for producing packets onto AF_XDP Tx ring
+consisting of multiple frames:
+
+* When an invalid descriptor is found, all the other
+  descriptors/frames of this packet are marked as invalid and not
+  completed. The next descriptor is treated as the start of a new
+  packet, even if this was not the intent (because we cannot guess
+  the intent). As before, if your program is producing invalid
+  descriptors you have a bug that must be fixed.
+
+* Zero length descriptors are treated as invalid descriptors.
+
+* For copy mode, the maximum supported number of frames in a packet is
+  equal to CONFIG_MAX_SKB_FRAGS + 1. If it is exceeded, all
+  descriptors accumulated so far are dropped and treated as
+  invalid. To produce an application that will work on any system
+  regardless of this config setting, limit the number of frags to 18,
+  as the minimum value of the config is 17.
+
+* For zero-copy mode, the limit is up to what the NIC HW
+  supports. Usually at least five on the NICs we have checked. We
+  consciously chose to not enforce a rigid limit (such as
+  CONFIG_MAX_SKB_FRAGS + 1) for zero-copy mode, as it would have
+  resulted in copy actions under the hood to fit into what limit the
+  NIC supports. Kind of defeats the purpose of zero-copy mode. How to
+  probe for this limit is explained in the "probe for multi-buffer
+  support" section.
+
+On the Rx path in copy-mode, the xsk core copies the XDP data into
+multiple descriptors, if needed, and sets the XDP_PKT_CONTD flag as
+detailed before. Zero-copy mode works the same, though the data is not
+copied. When the application gets a descriptor with the XDP_PKT_CONTD
+flag set to one, it means that the packet consists of multiple buffers
+and it continues with the next buffer in the following
+descriptor. When a descriptor with XDP_PKT_CONTD == 0 is received, it
+means that this is the last buffer of the packet. AF_XDP guarantees
+that only a complete packet (all frames in the packet) is sent to the
+application. If there is not enough space in the AF_XDP Rx ring, all
+frames of the packet will be dropped.
+
+If application reads a batch of descriptors, using for example the libxdp
+interfaces, it is not guaranteed that the batch will end with a full
+packet. It might end in the middle of a packet and the rest of the
+buffers of that packet will arrive at the beginning of the next batch,
+since the libxdp interface does not read the whole ring (unless you
+have an enormous batch size or a very small ring size).
+
+An example program each for Rx and Tx multi-buffer support can be found
+later in this document.
+
 Usage
-=====
+-----
 
 In order to use AF_XDP sockets two parts are needed. The
 user-space application and the XDP program. For a complete setup and
@@ -541,6 +625,131 @@ like this:
 But please use the libbpf functions as they are optimized and ready to
 use. Will make your life easier.
 
+Usage Multi-Buffer Rx
+---------------------
+
+Here is a simple Rx path pseudo-code example (using libxdp interfaces
+for simplicity). Error paths have been excluded to keep it short:
+
+.. code-block:: c
+
+    void rx_packets(struct xsk_socket_info *xsk)
+    {
+        static bool new_packet = true;
+        u32 idx_rx = 0, idx_fq = 0;
+        static char *pkt;
+
+        int rcvd = xsk_ring_cons__peek(&xsk->rx, opt_batch_size, &idx_rx);
+
+        xsk_ring_prod__reserve(&xsk->umem->fq, rcvd, &idx_fq);
+
+        for (int i = 0; i < rcvd; i++) {
+            struct xdp_desc *desc = xsk_ring_cons__rx_desc(&xsk->rx, idx_rx++);
+            char *frag = xsk_umem__get_data(xsk->umem->buffer, desc->addr);
+            bool eop = !(desc->options & XDP_PKT_CONTD);
+
+            if (new_packet)
+                pkt = frag;
+            else
+                add_frag_to_pkt(pkt, frag);
+
+            if (eop)
+                process_pkt(pkt);
+
+            new_packet = eop;
+
+            *xsk_ring_prod__fill_addr(&xsk->umem->fq, idx_fq++) = desc->addr;
+        }
+
+        xsk_ring_prod__submit(&xsk->umem->fq, rcvd);
+        xsk_ring_cons__release(&xsk->rx, rcvd);
+    }
+
+Usage Multi-Buffer Tx
+---------------------
+
+Here is an example Tx path pseudo-code (using libxdp interfaces for
+simplicity) ignoring that the umem is finite in size, and that we
+eventually will run out of packets to send. Also assumes pkts.addr
+points to a valid location in the umem.
+
+.. code-block:: c
+
+    void tx_packets(struct xsk_socket_info *xsk, struct pkt *pkts,
+                    int batch_size)
+    {
+        u32 idx, i, pkt_nb = 0;
+
+        xsk_ring_prod__reserve(&xsk->tx, batch_size, &idx);
+
+        for (i = 0; i < batch_size;) {
+            u64 addr = pkts[pkt_nb].addr;
+            u32 len = pkts[pkt_nb].size;
+
+            do {
+                struct xdp_desc *tx_desc;
+
+                tx_desc = xsk_ring_prod__tx_desc(&xsk->tx, idx + i++);
+                tx_desc->addr = addr;
+
+                if (len > xsk_frame_size) {
+                    tx_desc->len = xsk_frame_size;
+                    tx_desc->options = XDP_PKT_CONTD;
+                } else {
+                    tx_desc->len = len;
+                    tx_desc->options = 0;
+                    pkt_nb++;
+                }
+                len -= tx_desc->len;
+                addr += xsk_frame_size;
+
+                if (i == batch_size) {
+                    /* Remember len, addr, pkt_nb for next iteration.
+                     * Skipped for simplicity.
+                     */
+                    break;
+                }
+            } while (len);
+        }
+
+        xsk_ring_prod__submit(&xsk->tx, i);
+    }
+
+Probing for Multi-Buffer Support
+--------------------------------
+
+To discover if a driver supports multi-buffer AF_XDP in SKB or DRV
+mode, use the XDP_FEATURES feature of netlink in linux/netdev.h to
+query for NETDEV_XDP_ACT_RX_SG support. This is the same flag as for
+querying for XDP multi-buffer support. If XDP supports multi-buffer in
+a driver, then AF_XDP will also support that in SKB and DRV mode.
+
+To discover if a driver supports multi-buffer AF_XDP in zero-copy
+mode, use XDP_FEATURES and first check the NETDEV_XDP_ACT_XSK_ZEROCOPY
+flag. If it is set, it means that at least zero-copy is supported and
+you should go and check the netlink attribute
+NETDEV_A_DEV_XDP_ZC_MAX_SEGS in linux/netdev.h. An unsigned integer
+value will be returned stating the max number of frags that are
+supported by this device in zero-copy mode. These are the possible
+return values:
+
+1: Multi-buffer for zero-copy is not supported by this device, as max
+   one fragment supported means that multi-buffer is not possible.
+
+>=2: Multi-buffer is supported in zero-copy mode for this device. The
+     returned number signifies the max number of frags supported.
+
+For an example on how these are used through libbpf, please take a
+look at tools/testing/selftests/bpf/xskxceiver.c.
+
+Multi-Buffer Support for Zero-Copy Drivers
+------------------------------------------
+
+Zero-copy drivers usually use the batched APIs for Rx and Tx
+processing. Note that the Tx batch API guarantees that it will provide
+a batch of Tx descriptors that ends with full packet at the end. This
+to facilitate extending a zero-copy driver with multi-buffer support.
+
 Sample application
 ==================
 

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

@@ -3585,11 +3585,6 @@ static int i40e_configure_rx_ring(struct i40e_ring *ring)
 	if (ring->xsk_pool) {
 		ring->rx_buf_len =
 		  xsk_pool_get_rx_frame_size(ring->xsk_pool);
-		/* For AF_XDP ZC, we disallow packets to span on
-		 * multiple buffers, thus letting us skip that
-		 * handling in the fast-path.
-		 */
-		chain_len = 1;
 		ret = xdp_rxq_info_reg_mem_model(&ring->xdp_rxq,
 						 MEM_TYPE_XSK_BUFF_POOL,
 						 NULL);
@@ -13822,6 +13817,7 @@ static int i40e_config_netdev(struct i40e_vsi *vsi)
 				       NETDEV_XDP_ACT_REDIRECT |
 				       NETDEV_XDP_ACT_XSK_ZEROCOPY |
 				       NETDEV_XDP_ACT_RX_SG;
+		netdev->xdp_zc_max_segs = I40E_MAX_BUFFER_TXD;
 	} else {
 		/* Relate the VSI_VMDQ name to the VSI_MAIN name. Note that we
 		 * are still limited by IFNAMSIZ, but we're adding 'v%d\0' to

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

@@ -2284,8 +2284,8 @@ static struct sk_buff *i40e_build_skb(struct i40e_ring *rx_ring,
  * If the buffer is an EOP buffer, this function exits returning false,
  * otherwise return true indicating that this is in fact a non-EOP buffer.
  */
-static bool i40e_is_non_eop(struct i40e_ring *rx_ring,
-			    union i40e_rx_desc *rx_desc)
+bool i40e_is_non_eop(struct i40e_ring *rx_ring,
+		     union i40e_rx_desc *rx_desc)
 {
 	/* if we are the last buffer then there is nothing else to do */
 #define I40E_RXD_EOF BIT(I40E_RX_DESC_STATUS_EOF_SHIFT)

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

@@ -473,6 +473,8 @@ int __i40e_maybe_stop_tx(struct i40e_ring *tx_ring, int size);
 bool __i40e_chk_linearize(struct sk_buff *skb);
 int i40e_xdp_xmit(struct net_device *dev, int n, struct xdp_frame **frames,
 		  u32 flags);
+bool i40e_is_non_eop(struct i40e_ring *rx_ring,
+		     union i40e_rx_desc *rx_desc);
 
 /**
  * i40e_get_head - Retrieve head from head writeback