Add documentation for io_uring zero copy Rx that explains requirements and the user API. Signed-off-by: David Wei <dw@xxxxxxxxxxx> --- Documentation/networking/iou-zcrx.rst | 201 ++++++++++++++++++++++++++ 1 file changed, 201 insertions(+) create mode 100644 Documentation/networking/iou-zcrx.rst diff --git a/Documentation/networking/iou-zcrx.rst b/Documentation/networking/iou-zcrx.rst new file mode 100644 index 000000000000..0a3af8c08c7e --- /dev/null +++ b/Documentation/networking/iou-zcrx.rst @@ -0,0 +1,201 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================== +io_uring zero copy Rx +===================== + +Introduction +============ + +io_uring zero copy Rx (ZC Rx) is a feature that removes kernel-to-user copy on +the network receive path, allowing packet data to be received directly into +userspace memory. This feature is different to TCP_ZEROCOPY_RECEIVE in that +there are no strict alignment requirements and no need to mmap()/munmap(). +Compared to kernel bypass solutions such as e.g. DPDK, the packet headers are +processed by the kernel TCP stack as normal. + +NIC HW Requirements +=================== + +Several NIC HW features are required for io_uring ZC Rx to work. For now the +kernel API does not configure the NIC and it must be done by the user. + +Header/data split +----------------- + +Required to split packets at the L4 boundary into a header and a payload. +Headers are received into kernel memory as normal and processed by the TCP +stack as normal. Payloads are received into userspace memory directly. + +Flow steering +------------- + +Specific HW Rx queues are configured for this feature, but modern NICs +typically distribute flows across all HW Rx queues. Flow steering is required +to ensure that only desired flows are directed towards HW queues that are +configured for io_uring ZC Rx. + +RSS +--- + +In addition to flow steering above, RSS is required to steer all other non-zero +copy flows away from queues that are configured for io_uring ZC Rx. + +Usage +===== + +Setup NIC +--------- + +Must be done out of band for now. + +Ensure there are enough queues:: + + ethtool -L eth0 combined 32 + +Enable header/data split:: + + ethtool -G eth0 tcp-data-split on + +Carve out half of the HW Rx queues for zero copy using RSS:: + + ethtool -X eth0 equal 16 + +Set up flow steering:: + + ethtool -N eth0 flow-type tcp6 ... action 16 + +Setup io_uring +-------------- + +This section describes the low level io_uring kernel API. Please refer to +liburing documentation for how to use the higher level API. + +Create an io_uring instance with the following required setup flags:: + + IORING_SETUP_SINGLE_ISSUER + IORING_SETUP_DEFER_TASKRUN + IORING_SETUP_CQE32 + +Create memory area +------------------ + +Allocate userspace memory area for receiving zero copy data:: + + void *area_ptr = mmap(NULL, area_size, + PROT_READ | PROT_WRITE, + MAP_ANONYMOUS | MAP_PRIVATE, + 0, 0); + +Create refill ring +------------------ + +Allocate memory for a shared ringbuf used for returning consumed buffers:: + + void *ring_ptr = mmap(NULL, ring_size, + PROT_READ | PROT_WRITE, + MAP_ANONYMOUS | MAP_PRIVATE, + 0, 0); + +This refill ring consists of some space for the header, followed by an array of +``struct io_uring_zcrx_rqe``:: + + size_t rq_entries = 4096; + size_t ring_size = rq_entries * sizeof(struct io_uring_zcrx_rqe) + PAGE_SIZE; + /* align to page size */ + ring_size = (ring_size + (PAGE_SIZE - 1)) & ~(PAGE_SIZE - 1); + +Register ZC Rx +-------------- + +Fill in registration structs:: + + struct io_uring_zcrx_area_reg area_reg = { + .addr = (__u64)(unsigned long)area_ptr, + .len = area_size, + .flags = 0, + }; + + struct io_uring_region_desc region_reg = { + .user_addr = (__u64)(unsigned long)ring_ptr, + .size = ring_size, + .flags = IORING_MEM_REGION_TYPE_USER, + }; + + struct io_uring_zcrx_ifq_reg reg = { + .if_idx = if_nametoindex("eth0"), + /* this is the HW queue with desired flow steered into it */ + .if_rxq = 16, + .rq_entries = rq_entries, + .area_ptr = (__u64)(unsigned long)&area_reg, + .region_ptr = (__u64)(unsigned long)®ion_reg, + }; + +Register with kernel:: + + io_uring_register_ifq(ring, ®); + +Map refill ring +--------------- + +The kernel fills in fields for the refill ring in the registration ``struct +io_uring_zcrx_ifq_reg``. Map it into userspace:: + + struct io_uring_zcrx_rq refill_ring; + + refill_ring.khead = (unsigned *)((char *)ring_ptr + reg.offsets.head); + refill_ring.khead = (unsigned *)((char *)ring_ptr + reg.offsets.tail); + refill_ring.rqes = + (struct io_uring_zcrx_rqe *)((char *)ring_ptr + reg.offsets.rqes); + refill_ring.rq_tail = 0; + refill_ring.ring_ptr = ring_ptr; + +Receiving data +-------------- + +Prepare a zero copy recv request:: + + struct io_uring_sqe *sqe; + + sqe = io_uring_get_sqe(ring); + io_uring_prep_rw(IORING_OP_RECV_ZC, sqe, fd, NULL, 0, 0); + sqe->ioprio |= IORING_RECV_MULTISHOT; + +Now, submit and wait:: + + io_uring_submit_and_wait(ring, 1); + +Finally, process completions:: + + struct io_uring_cqe *cqe; + unsigned int count = 0; + unsigned int head; + + io_uring_for_each_cqe(ring, head, cqe) { + struct io_uring_zcrx_cqe *rcqe = (struct io_uring_zcrx_cqe *)(cqe + 1); + + unsigned char *data = area_ptr + (rcqe->off & IORING_ZCRX_AREA_MASK); + /* do something with the data */ + + count++; + } + io_uring_cq_advance(ring, count); + +Recycling buffers +----------------- + +Return buffers back to the kernel to be used again:: + + struct io_uring_zcrx_rqe *rqe; + unsigned mask = refill_ring.ring_entries - 1; + rqe = &refill_ring.rqes[refill_ring.rq_tail & mask]; + + area_offset = rcqe->off & IORING_ZCRX_AREA_MASK; + rqe->off = area_offset | area_reg.rq_area_token; + rqe->len = cqe->res; + IO_URING_WRITE_ONCE(*refill_ring.ktail, ++refill_ring.rq_tail); + +Testing +======= + +See ``tools/testing/selftests/net/iou-zcrx.c`` -- 2.43.5