Message-ID: <ZhUBAAoqLtqysnRW@archie.me>
Date: Tue, 9 Apr 2024 15:49:04 +0700
From: Bagas Sanjaya <bagasdotme@...il.com>
To: Aurelien Aptel <aaptel@...dia.com>, linux-nvme@...ts.infradead.org,
	netdev@...r.kernel.org, sagi@...mberg.me, hch@....de,
	kbusch@...nel.org, axboe@...com, chaitanyak@...dia.com,
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Cc: Yoray Zack <yorayz@...dia.com>, aurelien.aptel@...il.com,
	smalin@...dia.com, malin1024@...il.com, ogerlitz@...dia.com,
	borisp@...dia.com, galshalom@...dia.com, mgurtovoy@...dia.com,
	linux-doc@...r.kernel.org, edumazet@...gle.com, pabeni@...hat.com,
	corbet@....net
Subject: Re: [PATCH v24 09/20] Documentation: add ULP DDP offload
 documentation

On Thu, Apr 04, 2024 at 12:37:06PM +0000, Aurelien Aptel wrote:
> diff --git a/Documentation/networking/ulp-ddp-offload.rst b/Documentation/networking/ulp-ddp-offload.rst
> new file mode 100644
> index 000000000000..4133e5094ff5
> --- /dev/null
> +++ b/Documentation/networking/ulp-ddp-offload.rst
> @@ -0,0 +1,372 @@
> +.. 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/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 driver sets the ULP DDP operations
> +for the :c:type:`struct net_device <net_device>` via
> +`netdev->netdev_ops->ulp_ddp_ops`.
> +
> +The :c:member:`get_caps` operation returns the ULP DDP capabilities
> +enabled and/or supported by the device to the caller. The current list
> +of capabilities is represented as a bitset:
> +
> +.. code-block:: c
> +
> +  enum ulp_ddp_cap {
> +	ULP_DDP_CAP_NVME_TCP,
> +	ULP_DDP_CAP_NVME_TCP_DDGST,
> +  };
> +
> +The enablement of capabilities can be controlled via the
> +:c:member:`set_caps` operation. This operation is exposed to userspace
> +via netlink. See Documentation/netlink/specs/ulp_ddp.yaml for more
> +details.
> +
> +Later, after the L5P completes its handshake, the L5P queries the
> +driver for its runtime limitations via the :c:member:`limits` operation:
> +
> +.. code-block:: c
> +
> + int (*limits)(struct net_device *netdev,
> +	       struct ulp_ddp_limits *lim);
> +
> +
> +All L5P share a common set of limits and parameters (:c:type:`struct ulp_ddp_limits <ulp_ddp_limits>`):
> +
> +.. code-block:: c
> +
> + /**
> +  * struct ulp_ddp_limits - Generic ulp ddp limits: tcp ddp
> +  * protocol limits.
> +  * Add new instances of ulp_ddp_limits in the union below (nvme-tcp, etc.).
> +  *
> +  * @type:		type of this limits struct
> +  * @max_ddp_sgl_len:	maximum sgl size supported (zero means no limit)
> +  * @io_threshold:	minimum payload size required to offload
> +  * @tls:		support for ULP over TLS
> +  * @nvmeotcp:		NVMe-TCP specific limits
> +  */
> + struct ulp_ddp_limits {
> +	enum ulp_ddp_type	type;
> +	int			max_ddp_sgl_len;
> +	int			io_threshold;
> +	bool			tls:1;
> +	union {
> +		/* ... protocol-specific limits ... */
> +		struct nvme_tcp_ddp_limits nvmeotcp;
> +	};
> + };
> +
> +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 {
> +	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:`sk_add`:
> +
> +.. code-block:: c
> +
> + int (*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
> +  */
> + struct nvme_tcp_ddp_config {
> +	u16			pfv;
> +	u8			cpda;
> +	u8			dgst;
> +	int			queue_size;
> + };
> +
> +When offload is not needed anymore, e.g. when the socket is being released, the L5P
> +calls :c:member:`sk_del` to release device contexts:
> +
> +.. code-block:: c
> +
> + void (*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:`setup` of :c:type:`struct ulp_ddp_dev_ops
> +<ulp_ddp_dev_ops>`:
> +
> +.. code-block:: c
> +
> + int (*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:`teardown` of :c:type:`struct
> +ulp_ddp_dev_ops <ulp_ddp_dev_ops>`:
> +
> +.. code-block:: c
> +
> + void (*teardown)(struct net_device *netdev,
> +		  struct sock *sk,
> +		  struct ulp_ddp_io *io,
> +		  void *ddp_ctx);
> +
> +`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 `teardown`. This operation may be
> +performed either synchronously or asynchronously. In asynchronous teardown,
> +`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 <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 `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 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 <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:`resync` function of
> +the :c:type:`struct ulp_ddp_dev_ops <ulp_ddp_ops>` inside the :c:type:`struct
> +net_device <net_device>` while passing the same `seq` to confirm it is a PDU
> +header.
> +
> +.. code-block:: c
> +
> + void (*resync)(struct net_device *netdev,
> +		struct sock *sk, u32 seq);
> +
> +Statistics
> +==========
> +
> +Per L5P protocol, the NIC driver must report statistics for the above
> +netdevice operations and packets processed by offload.
> +These statistics are per-device and can be retrieved from userspace
> +via netlink (see Documentation/netlink/specs/ulp_ddp.yaml).
> +
> +For example, NVMe-TCP offload reports:
> +
> + * ``rx_nvme_tcp_sk_add`` - number of NVMe-TCP Rx offload contexts created.
> + * ``rx_nvme_tcp_sk_add_fail`` - number of NVMe-TCP Rx offload context creation
> +   failures.
> + * ``rx_nvme_tcp_sk_del`` - number of NVMe-TCP Rx offload contexts destroyed.
> + * ``rx_nvme_tcp_setup`` - number of DDP buffers mapped.
> + * ``rx_nvme_tcp_setup_fail`` - number of DDP buffers mapping that failed.
> + * ``rx_nvme_tcp_teardown`` - number of DDP buffers unmapped.
> + * ``rx_nvme_tcp_drop`` - number of packets dropped in the driver due to fatal
> +   errors.
> + * ``rx_nvme_tcp_resync`` - number of packets with resync requests.
> + * ``rx_nvme_tcp_packets`` - number of packets that used offload.
> + * ``rx_nvme_tcp_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.

The doc LGTM, thanks!

Reviewed-by: Bagas Sanjaya <bagasdotme@...il.com>

-- 
An old man doll... just what I always wanted! - Clara

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