man/io_uring_setup.2

.\" Copyright (C) 2019 Jens Axboe <axboe@kernel.dk>
.\" Copyright (C) 2019 Jon Corbet <corbet@lwn.net>
.\" Copyright (C) 2019 Red Hat, Inc.
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH IO_URING_SETUP 2 2019-01-29 "Linux" "Linux Programmer's Manual"
.SH NAME
io_uring_setup \- setup a context for performing asynchronous I/O
.SH SYNOPSIS
.nf
.BR "#include <linux/io_uring.h>"
.PP
.BI "int io_uring_setup(u32 " entries ", struct io_uring_params *" p );
.fi
.PP
.SH DESCRIPTION
.PP
The io_uring_setup() system call sets up a submission queue (SQ) and
completion queue (CQ) with at least
.I 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.

.I params
is used by the application to pass options to the kernel, and by the
kernel to convey information about the ring buffers.
.PP
.in +4n
.EX
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;
};
.EE
.in
.PP
The
.IR flags ,
.IR sq_thread_cpu ,
and
.I sq_thread_idle
fields are used to configure the io_uring instance.
.I flags
is a bit mask of 0 or more of the following values ORed
together:
.TP
.B 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
.B 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
.BR io_uring_enter (2).
It is illegal to mix and match polled and non-polled I/O on an io_uring
instance.

.TP
.B 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
.I sq_thread_idle
milliseconds, it will set the
.B IORING_SQ_NEED_WAKEUP
bit in the
.I flags
field of the
.IR "struct io_sq_ring" .
When this happens, the application must call
.BR 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
.BR io_uring_enter (2)
call with the following code sequence:

.in +4n
.EX
/*
 * 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);
.EE
.in

where
.I sq_ring
is a submission queue ring setup using the
.I struct io_sqring_offsets
described below.
.TP
.BR
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
.BR io_uring_register (2)
using the
.B IORING_REGISTER_FILES
opcode. Failure to do so will result in submitted IO being errored with
.B EBADF.
The presence of this feature can be detected by the
.B 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
.B CAP_SYS_NICE
capability.
.TP
.B IORING_SETUP_SQ_AFF
If this flag is specified, then the poll thread will be bound to the
cpu set in the
.I sq_thread_cpu
field of the
.IR "struct io_uring_params" .
This flag is only meaningful when
.B IORING_SETUP_SQPOLL
is specified. When cgroup setting
.I cpuset.cpus
changes (typically in container environment), the bounded cpu set may be
changed as well.
.TP
.B IORING_SETUP_CQSIZE
Create the completion queue with
.IR "struct io_uring_params.cq_entries"
entries.  The value must be greater than
.IR entries ,
and may be rounded up to the next power-of-two.
.TP
.B IORING_SETUP_CLAMP
If this flag is specified, and if
.IR entries
exceeds
.B IORING_MAX_ENTRIES ,
then
.IR entries
will be clamped at
.B IORING_MAX_ENTRIES .
If the flag
.BR IORING_SETUP_SQPOLL
is set, and if the value of
.IR "struct io_uring_params.cq_entries"
exceeds
.B IORING_MAX_CQ_ENTRIES ,
then it will be clamped at
.B IORING_MAX_CQ_ENTRIES .
.TP
.B IORING_SETUP_ATTACH_WQ
This flag should be set in conjunction with 
.IR "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.
.TP
.B 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
.BR io_uring_register (2)
for details on how to enable the ring. Available since 5.10.
.PP
If no flags are specified, the io_uring instance is setup for
interrupt driven I/O.  I/O may be submitted using
.BR io_uring_enter (2)
and can be reaped by polling the completion queue.

The
.I resv
array must be initialized to zero.

.I features
is filled in by the kernel, which specifies various features supported
by current kernel version.
.TP
.B IORING_FEAT_SINGLE_MMAP
If this flag is set, the two SQ and CQ rings can be mapped with a single
.I mmap(2)
call. The SQEs must still be allocated separately. This brings the necessary
.I mmap(2)
calls down from three to two.
.TP
.B 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
.B -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.
.TP
.B 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.
.TP
.B IORING_FEAT_RW_CUR_POS
If this flag is set, applications can specify
.I offset
== -1 with
.B IORING_OP_{READV,WRITEV}
,
.B IORING_OP_{READ,WRITE}_FIXED
, and
.B IORING_OP_{READ,WRITE}
to mean current file position, which behaves like
.I preadv2(2)
and
.I pwritev2(2)
with
.I 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.
.TP
.B 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
.I
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
.I
io_uring_register(2)
with
.B IORING_REGISTER_PERSONALITY
and specify the personality to use in the sqe.
.TP
.B 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.
.TP
.B IORING_FEAT_POLL_32BITS
If this flag is set, the
.B IORING_OP_POLL_ADD
command accepts the full 32-bit range of epoll based flags. Most notably
.B EPOLLEXCLUSIVE
which allows exclusive (waking single waiters) behavior.
.TP
.B IORING_FEAT_SQPOLL_NONFIXED
If this flag is set, the
.B 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.

.PP
The rest of the fields in the
.I 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.
.I sq_entries
specifies the number of submission queue entries allocated.
.I sq_off
describes the offsets of various ring buffer fields:
.PP
.in +4n
.EX
struct io_sqring_offsets {
    __u32 head;
    __u32 tail;
    __u32 ring_mask;
    __u32 ring_entries;
    __u32 flags;
    __u32 dropped;
    __u32 array;
    __u32 resv[3];
};
.EE
.in
.PP
Taken together,
.I sq_entries
and
.I 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:
.PP
.in +4n
.EX
ptr = mmap(0, sq_off.array + sq_entries * sizeof(__u32),
           PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE,
           ring_fd, IORING_OFF_SQ_RING);
.EE
.in
.PP
where
.I sq_off
is the
.I io_sqring_offsets
structure, and
.I ring_fd
is the file descriptor returned from
.BR io_uring_setup (2).
The addition of
.I 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
.I sq_off.head
to the address returned from
.BR mmap (2):
.PP
.in +4n
.EX
head = ptr + sq_off.head;
.EE
.in

The
.I 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
.BR io_uring_enter (2)
is necessary.  See the documentation for the
.B IORING_SETUP_SQPOLL
flag above.
The
.I 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:
.PP
.in +4n
.EX
index = tail & ring_mask;
.EE
.in
.PP
The array of submission queue entries is mapped with:
.PP
.in +4n
.EX
sqentries = mmap(0, sq_entries * sizeof(struct io_uring_sqe),
                 PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE,
                 ring_fd, IORING_OFF_SQES);
.EE
.in
.PP
The completion queue is described by
.I cq_entries
and
.I cq_off
shown here:
.PP
.in +4n
.EX
struct io_cqring_offsets {
    __u32 head;
    __u32 tail;
    __u32 ring_mask;
    __u32 ring_entries;
    __u32 overflow;
    __u32 cqes;
    __u32 flags;
    __u32 resv[3];
};
.EE
.in
.PP
The completion queue is simpler, since the entries are not separated
from the queue itself, and can be mapped with:
.PP
.in +4n
.EX
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);
.EE
.in
.PP
Closing the file descriptor returned by
.BR io_uring_setup (2)
will free all resources associated with the io_uring context.
.PP
.SH RETURN VALUE
.BR io_uring_setup (2)
returns a new file descriptor on success.  The application may then
provide the file descriptor in a subsequent
.BR mmap (2)
call to map the submission and completion queues, or to the
.BR io_uring_register (2)
or
.BR io_uring_enter (2)
system calls.

On error, -1 is returned and
.I errno
is set appropriately.
.PP
.SH ERRORS
.TP
.B EFAULT
params is outside your accessible address space.
.TP
.B EINVAL
The resv array contains non-zero data, p.flags contains an unsupported
flag,
.I entries
is out of bounds,
.B IORING_SETUP_SQ_AFF
was specified, but
.B IORING_SETUP_SQPOLL
was not, or
.B IORING_SETUP_CQSIZE
was specified, but
.I io_uring_params.cq_entries
was invalid.
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been
reached (see the description of
.B RLIMIT_NOFILE
in
.BR getrlimit (2)).
.TP
.B ENFILE
The system-wide limit on the total number of open files has been
reached.
.TP
.B ENOMEM
Insufficient kernel resources are available.
.TP
.B EPERM
.B IORING_SETUP_SQPOLL
was specified, but the effective user ID of the caller did not have sufficient
privileges.
.SH SEE ALSO
.BR io_uring_register (2),
.BR io_uring_enter (2)