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SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause) + +================================= +ULP direct data placement offload +================================= + +Overview +======== + +The Linux kernel ULP direct data placement (DDP) offload infrastructure +provides tagged request-response protocols, such as NVMe-TCP, the ability to +place response data directly in pre-registered buffers according to header +tags. DDP is particularly useful for data-intensive pipelined protocols whose +responses may be reordered. + +For example, in NVMe-TCP numerous read requests are sent together and each +request is tagged using the PDU header CID field. Receiving servers process +requests as fast as possible and sometimes responses for smaller requests +bypasses responses to larger requests, e.g., 4KB reads bypass 1GB reads. +Thereafter, clients correlate responses to requests using PDU header CID tags. +The processing of each response requires copying data from SKBs to read +request destination buffers; The offload avoids this copy. The offload is +oblivious to destination buffers which can reside either in userspace +(O_DIRECT) or in kernel pagecache. + +Request TCP byte-stream: + +.. parsed-literal:: + + +---------------+-------+---------------+-------+---------------+-------+ + | PDU hdr CID=1 | Req 1 | PDU hdr CID=2 | Req 2 | PDU hdr CID=3 | Req 3 | + +---------------+-------+---------------+-------+---------------+-------+ + +Response TCP byte-stream: + +.. parsed-literal:: + + +---------------+--------+---------------+--------+---------------+--------+ + | PDU hdr CID=2 | Resp 2 | PDU hdr CID=3 | Resp 3 | PDU hdr CID=1 | Resp 1 | + +---------------+--------+---------------+--------+---------------+--------+ + +The driver builds SKB page fragments that point to destination buffers. +Consequently, SKBs represent the original data on the wire, which enables +*transparent* inter-operation with the network stack. To avoid copies between +SKBs and destination buffers, the layer-5 protocol (L5P) will check +``if (src == dst)`` for SKB page fragments, success indicates that data is +already placed there by NIC hardware and copy should be skipped. + +In addition, L5P might have DDGST which ensures data integrity over +the network. If not offloaded, ULP DDP might not be efficient as L5P +will need to go over the data and calculate it by itself, cancelling +out the benefits of the DDP copy skip. ULP DDP has support for Rx/Tx +DDGST offload. On the received side the NIC will verify DDGST for +received PDUs and update SKB->ulp_ddp and SKB->ulp_crc bits. If all the SKBs +making up a L5P PDU have crc on, L5P will skip on calculating and +verifying the DDGST for the corresponding PDU. On the Tx side, the NIC +will be responsible for calculating and filling the DDGST fields in +the sent PDUs. + +Offloading does require NIC hardware to track L5P protocol framing, similarly +to RX TLS offload (see documentation at +:ref:`Documentation/networking/tls-offload.rst `). NIC hardware +will parse PDU headers, extract fields such as operation type, length, tag +identifier, etc. and only offload segments that correspond to tags registered +with the NIC, see the :ref:`buf_reg` section. + +Device configuration +==================== + +During driver initialization the device sets the ``NETIF_F_HW_ULP_DDP`` feature +and installs its +:c:type:`struct ulp_ddp_ops ` +pointer in the :c:member:`ulp_ddp_ops` member of the +:c:type:`struct net_device `. + +Later, after the L5P completes its handshake, the L5P queries the +device driver for its ULP capabilities (:c:type:`enum ulp_ddp_offload_capabilities`) +and runtime limitations via the :c:member:`ulp_ddp_limits` callback: + +.. code-block:: c + + int (*ulp_ddp_limits)(struct net_device *netdev, + struct ulp_ddp_limits *limits); + +The current list of capabilities is: + +.. code-block:: c + + enum ulp_ddp_offload_capabilities { + ULP_DDP_C_NVME_TCP = 1, + ULP_DDP_C_NVME_TCP_DDGST_RX = 2, + }; + +All L5P share a common set of limits and parameters (:c:type:`struct ulp_ddp_limits`): + +.. code-block:: c + + /** + * struct ulp_ddp_limits - Generic ulp ddp limits: tcp ddp + * protocol limits. + * Protocol implementations must use this as the first member. + * Add new instances of ulp_ddp_limits below (nvme-tcp, etc.). + * + * @max_ddp_sgl_len: maximum sgl size supported (zero means no limit) + * @io_threshold: minimum payload size required to offload + */ + struct ulp_ddp_limits { + enum ulp_ddp_type type; + u64 offload_capabilities; + int max_ddp_sgl_len; + int io_threshold; + unsigned char buf[]; + }; + +But each L5P can also add protocol-specific limits e.g.: + +.. code-block:: c + + /** + * struct nvme_tcp_ddp_limits - nvme tcp driver limitations + * + * @full_ccid_range: true if the driver supports the full CID range + */ + struct nvme_tcp_ddp_limits { + struct ulp_ddp_limits lmt; + + bool full_ccid_range; + }; + +Once the L5P has made sure the device is supported the offload +operations are installed on the socket. + +If offload installation fails, then the connection is handled by software as if +offload was not attempted. + +To request offload for a socket `sk`, the L5P calls :c:member:`ulp_ddp_sk_add`: + +.. code-block:: c + + int (*ulp_ddp_sk_add)(struct net_device *netdev, + struct sock *sk, + struct ulp_ddp_config *config); + +The function return 0 for success. In case of failure, L5P software should +fallback to normal non-offloaded operations. The `config` parameter indicates +the L5P type and any metadata relevant for that protocol. For example, in +NVMe-TCP the following config is used: + +.. code-block:: c + + /** + * struct nvme_tcp_ddp_config - nvme tcp ddp configuration for an IO queue + * + * @pfv: pdu version (e.g., NVME_TCP_PFV_1_0) + * @cpda: controller pdu data alignment (dwords, 0's based) + * @dgst: digest types enabled. + * The netdev will offload crc if L5P data digest is supported. + * @queue_size: number of nvme-tcp IO queue elements + * @queue_id: queue identifier + * @cpu_io: cpu core running the IO thread for this queue + */ + struct nvme_tcp_ddp_config { + struct ulp_ddp_config cfg; + + u16 pfv; + u8 cpda; + u8 dgst; + int queue_size; + int queue_id; + int io_cpu; + }; + +When offload is not needed anymore, e.g. when the socket is being released, the L5P +calls :c:member:`ulp_ddp_sk_del` to release device contexts: + +.. code-block:: c + + void (*ulp_ddp_sk_del)(struct net_device *netdev, + struct sock *sk); + +Normal operation +================ + +At the very least, the device maintains the following state for each connection: + + * 5-tuple + * expected TCP sequence number + * mapping between tags and corresponding buffers + * current offset within PDU, PDU length, current PDU tag + +NICs should not assume any correlation between PDUs and TCP packets. +If TCP packets arrive in-order, offload will place PDU payloads +directly inside corresponding registered buffers. NIC offload should +not delay packets. If offload is not possible, than the packet is +passed as-is to software. To perform offload on incoming packets +without buffering packets in the NIC, the NIC stores some inter-packet +state, such as partial PDU headers. + +RX data-path +------------ + +After the device validates TCP checksums, it can perform DDP offload. The +packet is steered to the DDP offload context according to the 5-tuple. +Thereafter, the expected TCP sequence number is checked against the packet +TCP sequence number. If there is a match, offload is performed: the PDU payload +is DMA written to the corresponding destination buffer according to the PDU header +tag. The data should be DMAed only once, and the NIC receive ring will only +store the remaining TCP and PDU headers. + +We remark that a single TCP packet may have numerous PDUs embedded inside. NICs +can choose to offload one or more of these PDUs according to various +trade-offs. Possibly, offloading such small PDUs is of little value, and it is +better to leave it to software. + +Upon receiving a DDP offloaded packet, the driver reconstructs the original SKB +using page frags, while pointing to the destination buffers whenever possible. +This method enables seamless integration with the network stack, which can +inspect and modify packet fields transparently to the offload. + +.. _buf_reg: + +Destination buffer registration +------------------------------- + +To register the mapping between tags and destination buffers for a socket +`sk`, the L5P calls :c:member:`ulp_ddp_setup` of :c:type:`struct ulp_ddp_ops +`: + +.. code-block:: c + + int (*ulp_ddp_setup)(struct net_device *netdev, + struct sock *sk, + struct ulp_ddp_io *io); + + +The `io` provides the buffer via scatter-gather list (`sg_table`) and +corresponding tag (`command_id`): + +.. code-block:: c + /** + * struct ulp_ddp_io - tcp ddp configuration for an IO request. + * + * @command_id: identifier on the wire associated with these buffers + * @nents: number of entries in the sg_table + * @sg_table: describing the buffers for this IO request + * @first_sgl: first SGL in sg_table + */ + struct ulp_ddp_io { + u32 command_id; + int nents; + struct sg_table sg_table; + struct scatterlist first_sgl[SG_CHUNK_SIZE]; + }; + +After the buffers have been consumed by the L5P, to release the NIC mapping of +buffers the L5P calls :c:member:`ulp_ddp_teardown` of :c:type:`struct +ulp_ddp_ops `: + +.. code-block:: c + + int (*ulp_ddp_teardown)(struct net_device *netdev, + struct sock *sk, + struct ulp_ddp_io *io, + void *ddp_ctx); + +`ulp_ddp_teardown` receives the same `io` context and an additional opaque +`ddp_ctx` that is used for asynchronous teardown, see the :ref:`async_release` +section. + +.. _async_release: + +Asynchronous teardown +--------------------- + +To teardown the association between tags and buffers and allow tag reuse NIC HW +is called by the NIC driver during `ulp_ddp_teardown`. This operation may be +performed either synchronously or asynchronously. In asynchronous teardown, +`ulp_ddp_teardown` returns immediately without unmapping NIC HW buffers. Later, +when the unmapping completes by NIC HW, the NIC driver will call up to L5P +using :c:member:`ddp_teardown_done` of :c:type:`struct ulp_ddp_ulp_ops`: + +.. code-block:: c + + void (*ddp_teardown_done)(void *ddp_ctx); + +The `ddp_ctx` parameter passed in `ddp_teardown_done` is the same on provided +in `ulp_ddp_teardown` and it is used to carry some context about the buffers +and tags that are released. + +Resync handling +=============== + +RX +-- +In presence of packet drops or network packet reordering, the device may lose +synchronization between the TCP stream and the L5P framing, and require a +resync with the kernel's TCP stack. When the device is out of sync, no offload +takes place, and packets are passed as-is to software. (resync is very similar +to TLS offload (see documentation at +:ref:`Documentation/networking/tls-offload.rst `) + +If only packets with L5P data are lost or reordered, then resynchronization may +be avoided by NIC HW that keeps tracking PDU headers. If, however, PDU headers +are reordered, then resynchronization is necessary. + +To resynchronize hardware during traffic, we use a handshake between hardware +and software. The NIC HW searches for a sequence of bytes that identifies L5P +headers (i.e., magic pattern). For example, in NVMe-TCP, the PDU operation +type can be used for this purpose. Using the PDU header length field, the NIC +HW will continue to find and match magic patterns in subsequent PDU headers. If +the pattern is missing in an expected position, then searching for the pattern +starts anew. + +The NIC will not resume offload when the magic pattern is first identified. +Instead, it will request L5P software to confirm that indeed this is a PDU +header. To request confirmation the NIC driver calls up to L5P using +:c:member:`*resync_request` of :c:type:`struct ulp_ddp_ulp_ops`: + +.. code-block:: c + + bool (*resync_request)(struct sock *sk, u32 seq, u32 flags); + +The `seq` parameter contains the TCP sequence of the last byte in the PDU header. +The `flags` parameter contains a flag (`ULP_DDP_RESYNC_PENDING`) indicating whether +a request is pending or not. +L5P software will respond to this request after observing the packet containing +TCP sequence `seq` in-order. If the PDU header is indeed there, then L5P +software calls the NIC driver using the :c:member:`ulp_ddp_resync` function of +the :c:type:`struct ulp_ddp_ops ` inside the :c:type:`struct +net_device ` while passing the same `seq` to confirm it is a PDU +header. + +.. code-block:: c + + void (*ulp_ddp_resync)(struct net_device *netdev, + struct sock *sk, u32 seq); + +Statistics +========== + +Per L5P protocol, the following NIC driver must report statistics for the above +netdevice operations and packets processed by offload. For example, NVMe-TCP +offload reports: + + * ``rx_nvmeotcp_sk_add`` - number of NVMe-TCP Rx offload contexts created. + * ``rx_nvmeotcp_sk_add_fail`` - number of NVMe-TCP Rx offload context creation + failures. + * ``rx_nvmeotcp_sk_del`` - number of NVMe-TCP Rx offload contexts destroyed. + * ``rx_nvmeotcp_ddp_setup`` - number of DDP buffers mapped. + * ``rx_nvmeotcp_ddp_setup_fail`` - number of DDP buffers mapping that failed. + * ``rx_nvmeoulp_ddp_teardown`` - number of DDP buffers unmapped. + * ``rx_nvmeotcp_drop`` - number of packets dropped in the driver due to fatal + errors. + * ``rx_nvmeotcp_resync`` - number of packets with resync requests. + * ``rx_nvmeotcp_offload_packets`` - number of packets that used offload. + * ``rx_nvmeotcp_offload_bytes`` - number of bytes placed in DDP buffers. + +NIC requirements +================ + +NIC hardware should meet the following requirements to provide this offload: + + * Offload must never buffer TCP packets. + * Offload must never modify TCP packet headers. + * Offload must never reorder TCP packets within a flow. + * Offload must never drop TCP packets. + * Offload must not depend on any TCP fields beyond the + 5-tuple and TCP sequence number.