IO_URING_SETUP(2) Linux Programmer's Manual IO_URING_SETUP(2)

io_uring_setup - setup a context for performing asynchronous I/O

#include <linux/io_uring.h>
int io_uring_setup(u32 entries, struct io_uring_params *p);

The io_uring_setup() system call sets up a submission queue (SQ) and completion queue (CQ) with at least entries entries, and returns a file descriptor which can be used to perform subsequent operations on the io_uring instance. The submission and completion queues are shared between userspace and the kernel, which eliminates the need to copy data when initiating and completing I/O.

params is used by the application to pass options to the kernel, and by the kernel to convey information about the ring buffers.


struct io_uring_params {
    __u32 sq_entries;
    __u32 cq_entries;
    __u32 flags;
    __u32 sq_thread_cpu;
    __u32 sq_thread_idle;
    __u32 features;
    __u32 resv[4];
    struct io_sqring_offsets sq_off;
    struct io_cqring_offsets cq_off;
};

The flags, sq_thread_cpu, and sq_thread_idle fields are used to configure the io_uring instance. flags is a bit mask of 0 or more of the following values ORed together:

IORING_SETUP_IOPOLL
Perform busy-waiting for an I/O completion, as opposed to getting notifications via an asynchronous IRQ (Interrupt Request). The file system (if any) and block device must support polling in order for this to work. Busy-waiting provides lower latency, but may consume more CPU resources than interrupt driven I/O. Currently, this feature is usable only on a file descriptor opened using the O_DIRECT flag. When a read or write is submitted to a polled context, the application must poll for completions on the CQ ring by calling io_uring_enter(2). It is illegal to mix and match polled and non-polled I/O on an io_uring instance.
IORING_SETUP_SQPOLL
When this flag is specified, a kernel thread is created to perform submission queue polling. An io_uring instance configured in this way enables an application to issue I/O without ever context switching into the kernel. By using the submission queue to fill in new submission queue entries and watching for completions on the completion queue, the application can submit and reap I/Os without doing a single system call.

If the kernel thread is idle for more than sq_thread_idle milliseconds, it will set the IORING_SQ_NEED_WAKEUP bit in the flags field of the struct io_sq_ring. When this happens, the application must call io_uring_enter(2) to wake the kernel thread. If I/O is kept busy, the kernel thread will never sleep. An application making use of this feature will need to guard the io_uring_enter(2) call with the following code sequence:


/*
 * Ensure that the wakeup flag is read after the tail pointer
 * has been written. It's important to use memory load acquire
 * semantics for the flags read, as otherwise the application
 * and the kernel might not agree on the consistency of the
 * wakeup flag.
 */
unsigned flags = atomic_load_relaxed(sq_ring->flags);
if (flags & IORING_SQ_NEED_WAKEUP)
    io_uring_enter(fd, 0, 0, IORING_ENTER_SQ_WAKEUP);


where sq_ring is a submission queue ring setup using the struct io_sqring_offsets described below.

Before version 5.11 of the Linux kernel, to successfully use this feature, the application must register a set of files to be used for IO through io_uring_register(2) using the IORING_REGISTER_FILES opcode. Failure to do so will result in submitted IO being errored with EBADF. The presence of this feature can be detected by the IORING_FEAT_SQPOLL_NONFIXED feature flag. In version 5.11 and later, it is no longer necessary to register files to use this feature. 5.11 also allows using this as non-root, if the user has the CAP_SYS_NICE capability.
IORING_SETUP_SQ_AFF
If this flag is specified, then the poll thread will be bound to the cpu set in the sq_thread_cpu field of the struct io_uring_params. This flag is only meaningful when IORING_SETUP_SQPOLL is specified.
IORING_SETUP_CQSIZE
Create the completion queue with struct io_uring_params.cq_entries entries. The value must be greater than entries, and may be rounded up to the next power-of-two.
IORING_SETUP_CLAMP
If this flag is specified, and if entries exceeds IORING_MAX_ENTRIES , then entries will be clamped at IORING_MAX_ENTRIES . If the flag IORING_SETUP_SQPOLL is set, and if the value of struct io_uring_params.cq_entries exceeds IORING_MAX_CQ_ENTRIES , then it will be clamped at IORING_MAX_CQ_ENTRIES .
IORING_SETUP_ATTACH_WQ
This flag should be set in conjunction with struct io_uring_params.wq_fd being set to an existing io_uring ring file descriptor. When set, the io_uring instance being created will share the asynchronous worker thread backend of the specified io_uring ring, rather than create a new separate thread pool.
IORING_SETUP_R_DISABLED
If this flag is specified, the io_uring ring starts in a disabled state. In this state, restrictions can be registered, but submissions are not allowed. See io_uring_register(2) for details on how to enable the ring. Available since 5.10.

If no flags are specified, the io_uring instance is setup for interrupt driven I/O. I/O may be submitted using io_uring_enter(2) and can be reaped by polling the completion queue.

The resv array must be initialized to zero.

features is filled in by the kernel, which specifies various features supported by current kernel version.

IORING_FEAT_SINGLE_MMAP
If this flag is set, the two SQ and CQ rings can be mapped with a single mmap(2) call. The SQEs must still be allocated separately. This brings the necessary mmap(2) calls down from three to two.
IORING_FEAT_NODROP
If this flag is set, io_uring supports never dropping completion events. If a completion event occurs and the CQ ring is full, the kernel stores the event internally until such a time that the CQ ring has room for more entries. If this overflow condition is entered, attempting to submit more IO with fail with the -EBUSY error value, if it can't flush the overflown events to the CQ ring. If this happens, the application must reap events from the CQ ring and attempt the submit again.
IORING_FEAT_SUBMIT_STABLE
If this flag is set, applications can be certain that any data for async offload has been consumed when the kernel has consumed the SQE.
IORING_FEAT_RW_CUR_POS
If this flag is set, applications can specify offset == -1 with IORING_OP_{READV,WRITEV} , IORING_OP_{READ,WRITE}_FIXED , and IORING_OP_{READ,WRITE} to mean current file position, which behaves like preadv2(2) and pwritev2(2) with offset == -1. It'll use (and update) the current file position. This obviously comes with the caveat that if the application has multiple reads or writes in flight, then the end result will not be as expected. This is similar to threads sharing a file descriptor and doing IO using the current file position.
IORING_FEAT_CUR_PERSONALITY
If this flag is set, then io_uring guarantees that both sync and async execution of a request assumes the credentials of the task that called io_uring_enter(2) to queue the requests. If this flag isn't set, then requests are issued with the credentials of the task that originally registered the io_uring. If only one task is using a ring, then this flag doesn't matter as the credentials will always be the same. Note that this is the default behavior, tasks can still register different personalities through io_uring_register(2) with IORING_REGISTER_PERSONALITY and specify the personality to use in the sqe.
IORING_FEAT_FAST_POLL
If this flag is set, then io_uring supports using an internal poll mechanism to drive data/space readiness. This means that requests that cannot read or write data to a file no longer need to be punted to an async thread for handling, instead they will begin operation when the file is ready. This is similar to doing poll + read/write in userspace, but eliminates the need to do so. If this flag is set, requests waiting on space/data consume a lot less resources doing so as they are not blocking a thread.
IORING_FEAT_POLL_32BITS
If this flag is set, the IORING_OP_POLL_ADD command accepts the full 32-bit range of epoll based flags. Most notably EPOLLEXCLUSIVE which allows exclusive (waking single waiters) behavior.
IORING_FEAT_SQPOLL_NONFIXED
If this flag is set, the IORING_SETUP_SQPOLL feature no longer requires the use of fixed files. Any normal file descriptor can be used for IO commands without needing registration.

The rest of the fields in the struct io_uring_params are filled in by the kernel, and provide the information necessary to memory map the submission queue, completion queue, and the array of submission queue entries. sq_entries specifies the number of submission queue entries allocated. sq_off describes the offsets of various ring buffer fields:


struct io_sqring_offsets {
    __u32 head;
    __u32 tail;
    __u32 ring_mask;
    __u32 ring_entries;
    __u32 flags;
    __u32 dropped;
    __u32 array;
    __u32 resv[3];
};

Taken together, sq_entries and sq_off provide all of the information necessary for accessing the submission queue ring buffer and the submission queue entry array. The submission queue can be mapped with a call like:


ptr = mmap(0, sq_off.array + sq_entries * sizeof(__u32),
           PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE,
           ring_fd, IORING_OFF_SQ_RING);

where sq_off is the io_sqring_offsets structure, and ring_fd is the file descriptor returned from io_uring_setup(2). The addition of sq_off.array to the length of the region accounts for the fact that the ring located at the end of the data structure. As an example, the ring buffer head pointer can be accessed by adding sq_off.head to the address returned from mmap(2):


head = ptr + sq_off.head;

The flags field is used by the kernel to communicate state information to the application. Currently, it is used to inform the application when a call to io_uring_enter(2) is necessary. See the documentation for the IORING_SETUP_SQPOLL flag above. The dropped member is incremented for each invalid submission queue entry encountered in the ring buffer.

The head and tail track the ring buffer state. The tail is incremented by the application when submitting new I/O, and the head is incremented by the kernel when the I/O has been successfully submitted. Determining the index of the head or tail into the ring is accomplished by applying a mask:


index = tail & ring_mask;

The array of submission queue entries is mapped with:


sqentries = mmap(0, sq_entries * sizeof(struct io_uring_sqe),
                 PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE,
                 ring_fd, IORING_OFF_SQES);

The completion queue is described by cq_entries and cq_off shown here:


struct io_cqring_offsets {
    __u32 head;
    __u32 tail;
    __u32 ring_mask;
    __u32 ring_entries;
    __u32 overflow;
    __u32 cqes;
    __u32 flags;
    __u32 resv[3];
};

The completion queue is simpler, since the entries are not separated from the queue itself, and can be mapped with:


ptr = mmap(0, cq_off.cqes + cq_entries * sizeof(struct io_uring_cqe),
           PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, ring_fd,
           IORING_OFF_CQ_RING);

Closing the file descriptor returned by io_uring_setup(2) will free all resources associated with the io_uring context.

io_uring_setup(2) returns a new file descriptor on success. The application may then provide the file descriptor in a subsequent mmap(2) call to map the submission and completion queues, or to the io_uring_register(2) or io_uring_enter(2) system calls.

On error, -1 is returned and errno is set appropriately.

EFAULT
params is outside your accessible address space.
EINVAL
The resv array contains non-zero data, p.flags contains an unsupported flag, entries is out of bounds, IORING_SETUP_SQ_AFF was specified, but IORING_SETUP_SQPOLL was not, or IORING_SETUP_CQSIZE was specified, but io_uring_params.cq_entries was invalid.
EMFILE
The per-process limit on the number of open file descriptors has been reached (see the description of RLIMIT_NOFILE in getrlimit(2)).
ENFILE
The system-wide limit on the total number of open files has been reached.
ENOMEM
Insufficient kernel resources are available.
EPERM
IORING_SETUP_SQPOLL was specified, but the effective user ID of the caller did not have sufficient privileges.

io_uring_register(2), io_uring_enter(2)
2019-01-29 Linux