Ubuntu Manpage: io_uring_setup - setup a context for performing asynchronous I/O

NAME

       io_uring_setup - setup a context for performing asynchronous I/O

SYNOPSIS

       #include <liburing.h>

       int io_uring_setup(u32 entries, struct io_uring_params *p);

DESCRIPTION

The io_uring_setup(2) 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 wq_fd;
__u32 resv[3];
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.

This is only applicable for storage devices for now, and the storage device must be configured for
polling. How to do that depends on the device type in question. For NVMe devices, the nvme driver
must be loaded with the poll_queues parameter set to the desired number of polling queues. The
polling queues will be shared appropriately between the CPUs in the system, if the number is less
than the number of online CPU threads.

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.

Note that, when using a ring setup with IORING_SETUP_SQPOLL, you never directly call the
io_uring_enter(2) system call. That is usually taken care of by liburing's io_uring_submit(3)
function. It automatically determines if you are using polling mode or not and deals with when
your program needs to call io_uring_enter(2) without you having to bother about it.

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. In 5.13 this
requirement was also relaxed, and no special privileges are needed for SQPOLL in newer kernels.
Certain stable kernels older than 5.13 may also support unprivileged SQPOLL.

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. When cgroup setting cpuset.cpus changes (typically in container environment), the
bounded cpu set may be changed as well.

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_CQSIZE 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.

IORING_SETUP_SUBMIT_ALL
Normally io_uring stops submitting a batch of requests, if one of these requests results in an
error. This can cause submission of less than what is expected, if a request ends in error while
being submitted. If the ring is created with this flag, io_uring_enter(2) will continue submitting
requests even if it encounters an error submitting a request. CQEs are still posted for errored
request regardless of whether or not this flag is set at ring creation time, the only difference
is if the submit sequence is halted or continued when an error is observed. Available since 5.18.

IORING_SETUP_COOP_TASKRUN
By default, io_uring will interrupt a task running in userspace when a completion event comes in.
This is to ensure that completions run in a timely manner. For a lot of use cases, this is
overkill and can cause reduced performance from both the inter-processor interrupt used to do
this, the kernel/user transition, the needless interruption of the tasks userspace activities, and
reduced batching if completions come in at a rapid rate. Most applications don't need the forceful
interruption, as the events are processed at any kernel/user transition. The exception are setups
where the application uses multiple threads operating on the same ring, where the application
waiting on completions isn't the one that submitted them. For most other use cases, setting this
flag will improve performance. Available since 5.19.

IORING_SETUP_TASKRUN_FLAG
Used in conjunction with IORING_SETUP_COOP_TASKRUN, this provides a flag, IORING_SQ_TASKRUN, which
is set in the SQ ring flags whenever completions are pending that should be processed. liburing
will check for this flag even when doing io_uring_peek_cqe(3) and enter the kernel to process
them, and applications can do the same. This makes IORING_SETUP_TASKRUN_FLAG safe to use even when
applications rely on a peek style operation on the CQ ring to see if anything might be pending to
reap. Available since 5.19.

IORING_SETUP_SQE128
If set, io_uring will use 128-byte SQEs rather than the normal 64-byte sized variant. This is a
requirement for using certain request types, as of 5.19 only the IORING_OP_URING_CMD passthrough
command for NVMe passthrough needs this. Available since 5.19.

IORING_SETUP_CQE32
If set, io_uring will use 32-byte CQEs rather than the normal 16-byte sized variant. This is a
requirement for using certain request types, as of 5.19 only the IORING_OP_URING_CMD passthrough
command for NVMe passthrough needs this. Available since 5.19.

IORING_SETUP_SINGLE_ISSUER
A hint to the kernel that only a single task (or thread) will submit requests, which is used for
internal optimisations. The submission task is either the task that created the ring, or if
IORING_SETUP_R_DISABLED is specified then it is the task that enables the ring through
io_uring_register(2). The kernel enforces this rule, failing requests with -EEXIST if the
restriction is violated. Note that when IORING_SETUP_SQPOLL is set it is considered that the
polling task is doing all submissions on behalf of the userspace and so it always complies with
the rule disregarding how many userspace tasks do io_uring_enter(2). Available since 6.0.

IORING_SETUP_DEFER_TASKRUN
By default, io_uring will process all outstanding work at the end of any system call or thread
interrupt. This can delay the application from making other progress. Setting this flag will hint
to io_uring that it should defer work until an io_uring_enter(2) call with the
IORING_ENTER_GETEVENTS flag set. This allows the application to request work to run just before it
wants to process completions. This flag requires the IORING_SETUP_SINGLE_ISSUER flag to be set,
and also enforces that the call to io_uring_enter(2) is called from the same thread that submitted
requests. Note that if this flag is set then it is the application's responsibility to
periodically trigger work (for example via any of the CQE waiting functions) or else completions
may not be delivered. Available since 6.1.

IORING_SETUP_NO_MMAP
By default, io_uring allocates kernel memory that callers must subsequently mmap(2). If this flag
is set, io_uring instead uses caller-allocated buffers; p->cq_off.user_addr must point to the
memory for the sq/cq rings, and p->sq_off.user_addr must point to the memory for the sqes. Each
allocation must be contiguous memory. Typically, callers should allocate this memory by using
mmap(2) to allocate a huge page. If this flag is set, a subsequent attempt to mmap(2) the
io_uring file descriptor will fail. Available since 6.5.

IORING_SETUP_REGISTERED_FD_ONLY
If this flag is set, io_uring will register the ring file descriptor, and return the registered
descriptor index, without ever allocating an unregistered file descriptor. The caller will need to
use IORING_REGISTER_USE_REGISTERED_RING when calling io_uring_register(2). This flag only makes
sense when used alongside with IORING_SETUP_NO_MMAP, which also needs to be set. Available since
6.5.

IORING_SETUP_NO_SQARRAY
If this flag is set, entries in the submission queue will be submitted in order, wrapping around
to the first entry after reaching the end of the queue. In other words, there will be no more
indirection via the array of submission entries, and the queue will be indexed directly by the
submission queue tail and the range of indexed represented by it modulo queue size. Subsequently,
the user should not map the array of submission queue entries, and the corresponding offset in
struct io_sqring_offsets will be set to zero. Available since 6.6.

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. Available since kernel 5.4.

IORING_FEAT_NODROP
If this flag is set, io_uring supports almost never dropping completion events. A dropped event
can only occur if the kernel runs out of memory, in which case you have worse problems than a lost
event. Your application and others will likely get OOM killed anyway. 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. In earlier kernels, if this overflow condition is entered, attempting
to submit more IO would 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. If the kernel has no free memory to store the event internally it will be visible by
an increase in the overflow value on the cqring. Available since kernel 5.5. Additionally
io_uring_enter(2) will return -EBADR the next time it would otherwise sleep waiting for
completions (since kernel 5.19).

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. Available since kernel 5.5.

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. Available since kernel 5.6.

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. Available since kernel 5.6.

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. Available since kernel 5.7.

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.
Available since kernel 5.9.

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. Available
since kernel 5.11.

IORING_FEAT_ENTER_EXT_ARG
If this flag is set, then the io_uring_enter(2) system call supports passing in an extended
argument instead of just the sigset_t of earlier kernels. This. extended argument is of type
struct io_uring_getevents_arg and allows the caller to pass in both a sigset_t and a timeout
argument for waiting on events. The struct layout is as follows:

struct io_uring_getevents_arg {
__u64 sigmask;
__u32 sigmask_sz;
__u32 pad;
__u64 ts;
};

and a pointer to this struct must be passed in if IORING_ENTER_EXT_ARG is set in the flags for the
enter system call. Available since kernel 5.11.

IORING_FEAT_NATIVE_WORKERS
If this flag is set, io_uring is using native workers for its async helpers. Previous kernels
used kernel threads that assumed the identity of the original io_uring owning task, but later
kernels will actively create what looks more like regular process threads instead. Available since
kernel 5.12.

IORING_FEAT_RSRC_TAGS
If this flag is set, then io_uring supports a variety of features related to fixed files and
buffers. In particular, it indicates that registered buffers can be updated in-place, whereas
before the full set would have to be unregistered first. Available since kernel 5.13.

IORING_FEAT_CQE_SKIP
If this flag is set, then io_uring supports setting IOSQE_CQE_SKIP_SUCCESS in the submitted SQE,
indicating that no CQE should be generated for this SQE if it executes normally. If an error
happens processing the SQE, a CQE with the appropriate error value will still be generated.
Available since kernel 5.17.

IORING_FEAT_LINKED_FILE
If this flag is set, then io_uring supports sane assignment of files for SQEs that have
dependencies. For example, if a chain of SQEs are submitted with IOSQE_IO_LINK, then kernels
without this flag will prepare the file for each link upfront. If a previous link opens a file
with a known index, eg if direct descriptors are used with open or accept, then file assignment
needs to happen post execution of that SQE. If this flag is set, then the kernel will defer file
assignment until execution of a given request is started. Available since kernel 5.17.

IORING_FEAT_REG_REG_RING
If this flag is set, then io_uring supports calling io_uring_register(2) using a registered ring
fd, via IORING_REGISTER_USE_REGISTERED_RING. Available since kernel 6.3.

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 resv1;
__u64 user_addr;
};

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 is 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 resv1;
__u64 user_addr;
};

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. Note that this may happen asynchronously within the kernel, so it is not guaranteed
that resources are freed immediately.

RETURN VALUE

       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, a negative error code is returned. The caller should not rely on errno variable.

ERRORS

       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.  IORING_SETUP_REGISTERED_FD_ONLY was
              specified, but IORING_SETUP_NO_MMAP was not.

       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.

       EPERM  /proc/sys/kernel/io_uring_disabled has the value 2, or it has the value 1 and the calling  process
              does not hold the CAP_SYS_ADMIN capability or is not a member of /proc/sys/kernel/io_uring_group.

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

SEE ALSO