libev/ev_iouring.c

/*
 * libev linux io_uring fd activity backend
 *
 * Copyright (c) 2019 Marc Alexander Lehmann <libev@schmorp.de>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modifica-
 * tion, are permitted provided that the following conditions are met:
 *
 *   1.  Redistributions of source code must retain the above copyright notice,
 *       this list of conditions and the following disclaimer.
 *
 *   2.  Redistributions in binary form must reproduce the above copyright
 *       notice, this list of conditions and the following disclaimer in the
 *       documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MER-
 * CHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO
 * EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPE-
 * CIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
 * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTH-
 * ERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * Alternatively, the contents of this file may be used under the terms of
 * the GNU General Public License ("GPL") version 2 or any later version,
 * in which case the provisions of the GPL are applicable instead of
 * the above. If you wish to allow the use of your version of this file
 * only under the terms of the GPL and not to allow others to use your
 * version of this file under the BSD license, indicate your decision
 * by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL. If you do not delete the
 * provisions above, a recipient may use your version of this file under
 * either the BSD or the GPL.
 */

/*
 * general notes about linux io_uring:
 *
 * a) it's the best interface I have seen so far. on linux.
 * b) best is not necessarily very good.
 * c) it's better than the aio mess, doesn't suffer from the fork problems
 *    of linux aio or epoll and so on and so on. and you could do event stuff
 *    without any syscalls. what's not to like?
 * d) ok, it's vastly more complex, but that's ok, really.
 * e) why 3 mmaps instead of one? one would be more space-efficient,
 *    and I can't see what benefit three would have (other than being
 *    somehow resizable/relocatable, but that's apparently not possible).
 *    (FIXME: newer kernels can use 2 mmaps only, need to look into this).
 * f) hmm, it's practiclaly undebuggable (gdb can't access the memory, and
 *    the bizarre way structure offsets are communicated makes it hard to
 *    just print the ring buffer heads, even *iff* the memory were visible
 *    in gdb. but then, that's also ok, really.
 * g) well, you cannot specify a timeout when waiting for events. no,
 *    seriously, the interface doesn't support a timeout. never seen _that_
 *    before. sure, you can use a timerfd, but that's another syscall
 *    you could have avoided. overall, this bizarre omission smells
 *    like a µ-optimisation by the io_uring author for his personal
 *    applications, to the detriment of everybody else who just wants
 *    an event loop. but, umm, ok, if that's all, it could be worse.
 *    (FIXME: jens mentioned timeout commands, need to investigate)
 * h) there is a hardcoded limit of 4096 outstanding events. okay,
 *    at least there is no arbitrary low system-wide limit...
 *    (FIXME: apparently, this was increased to 32768 in later kernels(
 * i) unlike linux aio, you *can* register more then the limit
 *    of fd events, and the kernel will "gracefully" signal an
 *    overflow, after which you could destroy and recreate the kernel
 *    state, a bit bigger, or fall back to e.g. poll. thats not
 *    totally insane, but kind of questions the point a high
 *    performance I/O framework when it doesn't really work
 *    under stress.
 *    (FIXME: iouring should no longer drop events, need to investigate)
 * j) but, oh my! is has exactly the same bugs as the linux aio backend,
 *    where some undocumented poll combinations just fail.
 *    so we need epoll AGAIN as a fallback. AGAIN! epoll!! and of course,
 *    this is completely undocumented, have I mantioned this already?
 * k) overall, the *API* itself is, I dare to say, not a total trainwreck.
 *    the big isuess with it are the bugs requiring epoll, which might
 *    or might not get fixed (do I hold my breath?).
 */

/* TODO: use internal TIMEOUT */
/* TODO: take advantage of single mmap, NODROP etc. */
/* TODO: resize cq/sq size independently */

#include <sys/timerfd.h>
#include <sys/mman.h>
#include <poll.h>

#define IOURING_INIT_ENTRIES 32

/*****************************************************************************/
/* syscall wrapdadoop - this section has the raw api/abi definitions */

#include <linux/fs.h>
#include <linux/types.h>

/* mostly directly taken from the kernel or documentation */

struct io_uring_sqe
{
  __u8 opcode;
  __u8 flags;
  __u16 ioprio;
  __s32 fd;
  union {
    __u64 off;
    __u64 addr2;
  };
  __u64 addr;
  __u32 len;
  union {
    __kernel_rwf_t rw_flags;
    __u32 fsync_flags;
    __u16 poll_events;
    __u32 sync_range_flags;
    __u32 msg_flags;
    __u32 timeout_flags;
    __u32 accept_flags;
    __u32 cancel_flags;
    __u32 open_flags;
    __u32 statx_flags;
  };
  __u64 user_data;
  union {
    __u16 buf_index;
    __u64 __pad2[3];
  };
};

struct io_uring_cqe
{
  __u64 user_data;
  __s32 res;
  __u32 flags;
};

struct io_sqring_offsets
{
  __u32 head;
  __u32 tail;
  __u32 ring_mask;
  __u32 ring_entries;
  __u32 flags;
  __u32 dropped;
  __u32 array;
  __u32 resv1;
  __u64 resv2;
};

struct io_cqring_offsets
{
  __u32 head;
  __u32 tail;
  __u32 ring_mask;
  __u32 ring_entries;
  __u32 overflow;
  __u32 cqes;
  __u64 resv[2];
};

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;
};

#define IORING_OP_POLL_ADD    6
#define IORING_OP_POLL_REMOVE 7

#define IORING_ENTER_GETEVENTS 0x01

#define IORING_OFF_SQ_RING 0x00000000ULL
#define IORING_OFF_CQ_RING 0x08000000ULL
#define IORING_OFF_SQES    0x10000000ULL

#define IORING_FEAT_SINGLE_MMAP   0x1
#define IORING_FEAT_NODROP        0x2
#define IORING_FEAT_SUBMIT_STABLE 0x4

inline_size
int
evsys_io_uring_setup (unsigned entries, struct io_uring_params *params)
{
  return ev_syscall2 (SYS_io_uring_setup, entries, params);
}

inline_size
int
evsys_io_uring_enter (int fd, unsigned to_submit, unsigned min_complete, unsigned flags, const sigset_t *sig, size_t sigsz)
{
  return ev_syscall6 (SYS_io_uring_enter, fd, to_submit, min_complete, flags, sig, sigsz);
}

/*****************************************************************************/
/* actual backed implementation */

/* we hope that volatile will make the compiler access this variables only once */
#define EV_SQ_VAR(name) *(volatile unsigned *)((char *)iouring_sq_ring + iouring_sq_ ## name)
#define EV_CQ_VAR(name) *(volatile unsigned *)((char *)iouring_cq_ring + iouring_cq_ ## name)

/* the index array */
#define EV_SQ_ARRAY     ((unsigned *)((char *)iouring_sq_ring + iouring_sq_array))

/* the submit/completion queue entries */
#define EV_SQES         ((struct io_uring_sqe *)         iouring_sqes)
#define EV_CQES         ((struct io_uring_cqe *)((char *)iouring_cq_ring + iouring_cq_cqes))

static
struct io_uring_sqe *
iouring_sqe_get (EV_P)
{
  unsigned tail = EV_SQ_VAR (tail);

  if (tail + 1 - EV_SQ_VAR (head) > EV_SQ_VAR (ring_entries))
    {
      /* queue full, flush */
      evsys_io_uring_enter (iouring_fd, iouring_to_submit, 0, 0, 0, 0);
      iouring_to_submit = 0;
    }

  assert (("libev: io_uring queue full after flush", tail + 1 - EV_SQ_VAR (head) <= EV_SQ_VAR (ring_entries)));

  return EV_SQES + (tail & EV_SQ_VAR (ring_mask));
}

inline_size
struct io_uring_sqe *
iouring_sqe_submit (EV_P_ struct io_uring_sqe *sqe)
{
  unsigned idx = sqe - EV_SQES;

  EV_SQ_ARRAY [idx] = idx;
  ECB_MEMORY_FENCE_RELEASE;
  ++EV_SQ_VAR (tail);
  /*ECB_MEMORY_FENCE_RELEASE; /* for the time being we assume this is not needed */
  ++iouring_to_submit;
}

/*****************************************************************************/

/* when the timerfd expires we simply note the fact,
 * as the purpose of the timerfd is to wake us up, nothing else.
 * the next iteration should re-set it.
 */
static void
iouring_tfd_cb (EV_P_ struct ev_io *w, int revents)
{
  iouring_tfd_to = EV_TSTAMP_HUGE;
}

/* called for full and partial cleanup */
ecb_cold
static int
iouring_internal_destroy (EV_P)
{
  close (iouring_tfd);
  close (iouring_fd);

  if (iouring_sq_ring != MAP_FAILED) munmap (iouring_sq_ring, iouring_sq_ring_size);
  if (iouring_cq_ring != MAP_FAILED) munmap (iouring_cq_ring, iouring_cq_ring_size);
  if (iouring_sqes    != MAP_FAILED) munmap (iouring_sqes   , iouring_sqes_size   );

  if (ev_is_active (&iouring_tfd_w))
    {
      ev_ref (EV_A);
      ev_io_stop (EV_A_ &iouring_tfd_w);
    }
}

ecb_cold
static int
iouring_internal_init (EV_P)
{
  struct io_uring_params params = { 0 };

  iouring_to_submit = 0;

  iouring_tfd     = -1;
  iouring_sq_ring = MAP_FAILED;
  iouring_cq_ring = MAP_FAILED;
  iouring_sqes    = MAP_FAILED;

  for (;;)
    {
      iouring_fd = evsys_io_uring_setup (iouring_entries, &params);

      if (iouring_fd >= 0)
        break; /* yippie */

      if (errno != EINVAL)
        return -1; /* we failed */

#if TODO
      if ((~params.features) & (IORING_FEAT_NODROP | IORING_FEATURE_SINGLE_MMAP))
        return -1; /* we require the above features */
#endif

      /* EINVAL: lots of possible reasons, but maybe
       * it is because we hit the unqueryable hardcoded size limit
       */

      /* we hit the limit already, give up */
      if (iouring_max_entries)
        return -1;

      /* first time we hit EINVAL? assume we hit the limit, so go back and retry */
      iouring_entries >>= 1;
      iouring_max_entries = iouring_entries;
    }

  iouring_sq_ring_size = params.sq_off.array + params.sq_entries * sizeof (unsigned);
  iouring_cq_ring_size = params.cq_off.cqes  + params.cq_entries * sizeof (struct io_uring_cqe);
  iouring_sqes_size    =                       params.sq_entries * sizeof (struct io_uring_sqe);

  iouring_sq_ring = mmap (0, iouring_sq_ring_size, PROT_READ | PROT_WRITE,
                          MAP_SHARED | MAP_POPULATE, iouring_fd, IORING_OFF_SQ_RING);
  iouring_cq_ring = mmap (0, iouring_cq_ring_size, PROT_READ | PROT_WRITE,
                          MAP_SHARED | MAP_POPULATE, iouring_fd, IORING_OFF_CQ_RING);
  iouring_sqes    = mmap (0, iouring_sqes_size, PROT_READ | PROT_WRITE,
                          MAP_SHARED | MAP_POPULATE, iouring_fd, IORING_OFF_SQES);

  if (iouring_sq_ring == MAP_FAILED || iouring_cq_ring == MAP_FAILED || iouring_sqes == MAP_FAILED)
    return -1;

  iouring_sq_head         = params.sq_off.head;
  iouring_sq_tail         = params.sq_off.tail;
  iouring_sq_ring_mask    = params.sq_off.ring_mask;
  iouring_sq_ring_entries = params.sq_off.ring_entries;
  iouring_sq_flags        = params.sq_off.flags;
  iouring_sq_dropped      = params.sq_off.dropped;
  iouring_sq_array        = params.sq_off.array;

  iouring_cq_head         = params.cq_off.head;
  iouring_cq_tail         = params.cq_off.tail;
  iouring_cq_ring_mask    = params.cq_off.ring_mask;
  iouring_cq_ring_entries = params.cq_off.ring_entries;
  iouring_cq_overflow     = params.cq_off.overflow;
  iouring_cq_cqes         = params.cq_off.cqes;

  iouring_tfd = timerfd_create (CLOCK_MONOTONIC, TFD_CLOEXEC);

  if (iouring_tfd < 0)
    return iouring_tfd;

  iouring_tfd_to = EV_TSTAMP_HUGE;

  return 0;
}

ecb_cold
static void
iouring_fork (EV_P)
{
  iouring_internal_destroy (EV_A);

  while (iouring_internal_init (EV_A) < 0)
    ev_syserr ("(libev) io_uring_setup");

  fd_rearm_all (EV_A);

  ev_io_stop  (EV_A_ &iouring_tfd_w);
  ev_io_set   (EV_A_ &iouring_tfd_w, iouring_tfd, EV_READ);
  ev_io_start (EV_A_ &iouring_tfd_w);
}

/*****************************************************************************/

static void
iouring_modify (EV_P_ int fd, int oev, int nev)
{
  if (oev)
    {
      /* we assume the sqe's are all "properly" initialised */
      struct io_uring_sqe *sqe = iouring_sqe_get (EV_A);
      sqe->opcode    = IORING_OP_POLL_REMOVE;
      sqe->fd        = fd;
      sqe->user_data = -1;
      iouring_sqe_submit (EV_A_ sqe);

      /* increment generation counter to avoid handling old events */
      ++anfds [fd].egen;
    }

  if (nev)
    {
      struct io_uring_sqe *sqe = iouring_sqe_get (EV_A);
      sqe->opcode      = IORING_OP_POLL_ADD;
      sqe->fd          = fd;
      sqe->user_data   = (uint32_t)fd | ((__u64)(uint32_t)anfds [fd].egen << 32);
      sqe->poll_events =
        (nev & EV_READ ? POLLIN : 0)
        | (nev & EV_WRITE ? POLLOUT : 0);
      iouring_sqe_submit (EV_A_ sqe);
    }
}

inline_size
void
iouring_tfd_update (EV_P_ ev_tstamp timeout)
{
  ev_tstamp tfd_to = mn_now + timeout;

  /* we assume there will be many iterations per timer change, so
   * we only re-set the timerfd when we have to because its expiry
   * is too late.
   */
  if (ecb_expect_false (tfd_to < iouring_tfd_to))
    {
       struct itimerspec its;

       iouring_tfd_to = tfd_to;
       EV_TS_SET (its.it_interval, 0.);
       EV_TS_SET (its.it_value, tfd_to);

       if (timerfd_settime (iouring_tfd, TFD_TIMER_ABSTIME, &its, 0) < 0)
         assert (("libev: iouring timerfd_settime failed", 0));
    }
}

inline_size
void
iouring_process_cqe (EV_P_ struct io_uring_cqe *cqe)
{
  int      fd  = cqe->user_data & 0xffffffffU;
  uint32_t gen = cqe->user_data >> 32;
  int      res = cqe->res;

  /* ignore fd removal events, if there are any. TODO: verify */
  /* TODO: yes, this triggers */
  if (cqe->user_data == (__u64)-1)
    return;

  assert (("libev: io_uring fd must be in-bounds", fd >= 0 && fd < anfdmax));

  /* documentation lies, of course. the result value is NOT like
   * normal syscalls, but like linux raw syscalls, i.e. negative
   * error numbers. fortunate, as otherwise there would be no way
   * to get error codes at all. still, why not document this?
   */

  /* ignore event if generation doesn't match */
  /* this should actually be very rare */
  if (ecb_expect_false (gen != (uint32_t)anfds [fd].egen))
    return;

  if (ecb_expect_false (res < 0))
    {
      /*TODO: EINVAL handling (was something failed with this fd)*/
      /*TODO: EBUSY happens when?*/

      if (res == -EBADF)
        {
          assert (("libev: event loop rejected bad fd", res != -EBADF));
          fd_kill (EV_A_ fd);
        }
      else
        {
          errno = -res;
          ev_syserr ("(libev) IORING_OP_POLL_ADD");
        }

      return;
    }

  /* feed events, we do not expect or handle POLLNVAL */
  fd_event (
    EV_A_
    fd,
    (res & (POLLOUT | POLLERR | POLLHUP) ? EV_WRITE : 0)
    | (res & (POLLIN | POLLERR | POLLHUP) ? EV_READ : 0)
  );

  /* io_uring is oneshot, so we need to re-arm the fd next iteration */
  /* this also means we usually have to do at least one syscall per iteration */
  anfds [fd].events = 0;
  fd_change (EV_A_ fd, EV_ANFD_REIFY);
}

/* called when the event queue overflows */
ecb_cold
static void
iouring_overflow (EV_P)
{
  /* we have two options, resize the queue (by tearing down
   * everything and recreating it, or living with it
   * and polling.
   * we implement this by resizing the queue, and, if that fails,
   * we just recreate the state on every failure, which
   * kind of is a very inefficient poll.
   * one danger is, due to the bios toward lower fds,
   * we will only really get events for those, so
   * maybe we need a poll() fallback, after all.
   */
  /*EV_CQ_VAR (overflow) = 0;*/ /* need to do this if we keep the state and poll manually */

  fd_rearm_all (EV_A);

  /* we double the size until we hit the hard-to-probe maximum */
  if (!iouring_max_entries)
    {
      iouring_entries <<= 1;
      iouring_fork (EV_A);
    }
  else
    {
      /* we hit the kernel limit, we should fall back to something else.
       * we can either poll() a few times and hope for the best,
       * poll always, or switch to epoll.
       * TODO: is this necessary with newer kernels?
       */

      iouring_internal_destroy (EV_A);

      /* this should make it so that on return, we don't call any uring functions */
      iouring_to_submit = 0;

      for (;;)
        {
          backend = epoll_init (EV_A_ 0);

          if (backend)
            break;

          ev_syserr ("(libev) iouring switch to epoll");
        }
    }
}

/* handle any events in the completion queue, return true if there were any */
static int
iouring_handle_cq (EV_P)
{
  unsigned head, tail, mask;
  
  head = EV_CQ_VAR (head);
  ECB_MEMORY_FENCE_ACQUIRE;
  tail = EV_CQ_VAR (tail);

  if (head == tail)
    return 0;

  /* it can only overflow if we have events, yes, yes? */
  if (ecb_expect_false (EV_CQ_VAR (overflow)))
    {
      iouring_overflow (EV_A);
      return 1;
    }

  mask = EV_CQ_VAR (ring_mask);

  do
    iouring_process_cqe (EV_A_ &EV_CQES [head++ & mask]);
  while (head != tail);

  EV_CQ_VAR (head) = head;
  ECB_MEMORY_FENCE_RELEASE;

  return 1;
}

static void
iouring_poll (EV_P_ ev_tstamp timeout)
{
  /* if we have events, no need for extra syscalls, but we might have to queue events */
  if (iouring_handle_cq (EV_A))
    timeout = EV_TS_CONST (0.);
  else
    /* no events, so maybe wait for some */
    iouring_tfd_update (EV_A_ timeout);

  /* only enter the kernel if we have something to submit, or we need to wait */
  if (timeout || iouring_to_submit)
    {
      int res;

      EV_RELEASE_CB;

      res = evsys_io_uring_enter (iouring_fd, iouring_to_submit, 1,
                                  timeout > EV_TS_CONST (0.) ? IORING_ENTER_GETEVENTS : 0, 0, 0);
      iouring_to_submit = 0;

      EV_ACQUIRE_CB;

      if (ecb_expect_false (res < 0))
        if (errno == EINTR)
          /* ignore */;
        else
          ev_syserr ("(libev) iouring setup");
      else
        iouring_handle_cq (EV_A);
    }
}

inline_size
int
iouring_init (EV_P_ int flags)
{
  iouring_entries     = IOURING_INIT_ENTRIES;
  iouring_max_entries = 0;

  if (iouring_internal_init (EV_A) < 0)
    {
      iouring_internal_destroy (EV_A);
      return 0;
    }

  ev_io_init  (&iouring_tfd_w, iouring_tfd_cb, iouring_tfd, EV_READ);
  ev_set_priority (&iouring_tfd_w, EV_MINPRI);
  ev_io_start (EV_A_ &iouring_tfd_w);
  ev_unref (EV_A); /* watcher should not keep loop alive */

  backend_modify = iouring_modify;
  backend_poll   = iouring_poll;

  return EVBACKEND_IOURING;
}

inline_size
void
iouring_destroy (EV_P)
{
  iouring_internal_destroy (EV_A);
}

Revision:	1.11
Committed:	Fri Dec 27 21:56:29 2019 UTC (4 years, 4 months ago) by root
Content type:	text/plain
Branch:	MAIN
Changes since 1.10:	+7 -2 lines
Log Message:	* empty log message *
#	Content
1	/*
2	* libev linux io_uring fd activity backend
3	*
4	* Copyright (c) 2019 Marc Alexander Lehmann <libev@schmorp.de>
5	* All rights reserved.
6	*
7	* Redistribution and use in source and binary forms, with or without modifica-
8	* tion, are permitted provided that the following conditions are met:
9	*
10	* 1. Redistributions of source code must retain the above copyright notice,
11	* this list of conditions and the following disclaimer.
12	*
13	* 2. Redistributions in binary form must reproduce the above copyright
14	* notice, this list of conditions and the following disclaimer in the
15	* documentation and/or other materials provided with the distribution.
16	*
17	* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
18	* WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MER-
19	* CHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
20	* EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPE-
21	* CIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
22	* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
23	* OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
24	* WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTH-
25	* ERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
26	* OF THE POSSIBILITY OF SUCH DAMAGE.
27	*
28	* Alternatively, the contents of this file may be used under the terms of
29	* the GNU General Public License ("GPL") version 2 or any later version,
30	* in which case the provisions of the GPL are applicable instead of
31	* the above. If you wish to allow the use of your version of this file
32	* only under the terms of the GPL and not to allow others to use your
33	* version of this file under the BSD license, indicate your decision
34	* by deleting the provisions above and replace them with the notice
35	* and other provisions required by the GPL. If you do not delete the
36	* provisions above, a recipient may use your version of this file under
37	* either the BSD or the GPL.
38	*/
39
40	/*
41	* general notes about linux io_uring:
42	*
43	* a) it's the best interface I have seen so far. on linux.
44	* b) best is not necessarily very good.
45	* c) it's better than the aio mess, doesn't suffer from the fork problems
46	* of linux aio or epoll and so on and so on. and you could do event stuff
47	* without any syscalls. what's not to like?
48	* d) ok, it's vastly more complex, but that's ok, really.
49	* e) why 3 mmaps instead of one? one would be more space-efficient,
50	* and I can't see what benefit three would have (other than being
51	* somehow resizable/relocatable, but that's apparently not possible).
52	* (FIXME: newer kernels can use 2 mmaps only, need to look into this).
53	* f) hmm, it's practiclaly undebuggable (gdb can't access the memory, and
54	* the bizarre way structure offsets are communicated makes it hard to
55	* just print the ring buffer heads, even iff the memory were visible
56	* in gdb. but then, that's also ok, really.
57	* g) well, you cannot specify a timeout when waiting for events. no,
58	* seriously, the interface doesn't support a timeout. never seen _that_
59	* before. sure, you can use a timerfd, but that's another syscall
60	* you could have avoided. overall, this bizarre omission smells
61	* like a µ-optimisation by the io_uring author for his personal
62	* applications, to the detriment of everybody else who just wants
63	* an event loop. but, umm, ok, if that's all, it could be worse.
64	* (FIXME: jens mentioned timeout commands, need to investigate)
65	* h) there is a hardcoded limit of 4096 outstanding events. okay,
66	* at least there is no arbitrary low system-wide limit...
67	* (FIXME: apparently, this was increased to 32768 in later kernels(
68	* i) unlike linux aio, you can register more then the limit
69	* of fd events, and the kernel will "gracefully" signal an
70	* overflow, after which you could destroy and recreate the kernel
71	* state, a bit bigger, or fall back to e.g. poll. thats not
72	* totally insane, but kind of questions the point a high
73	* performance I/O framework when it doesn't really work
74	* under stress.
75	* (FIXME: iouring should no longer drop events, need to investigate)
76	* j) but, oh my! is has exactly the same bugs as the linux aio backend,
77	* where some undocumented poll combinations just fail.
78	* so we need epoll AGAIN as a fallback. AGAIN! epoll!! and of course,
79	* this is completely undocumented, have I mantioned this already?
80	* k) overall, the API itself is, I dare to say, not a total trainwreck.
81	* the big isuess with it are the bugs requiring epoll, which might
82	* or might not get fixed (do I hold my breath?).
83	*/
84
85	/* TODO: use internal TIMEOUT */
86	/* TODO: take advantage of single mmap, NODROP etc. */
87	/* TODO: resize cq/sq size independently */
88
89	#include <sys/timerfd.h>
90	#include <sys/mman.h>
91	#include <poll.h>
92
93	#define IOURING_INIT_ENTRIES 32
94
95	/*****************************************************************************/
96	/* syscall wrapdadoop - this section has the raw api/abi definitions */
97
98	#include <linux/fs.h>
99	#include <linux/types.h>
100
101	/* mostly directly taken from the kernel or documentation */
102
103	struct io_uring_sqe
104	{
105	__u8 opcode;
106	__u8 flags;
107	__u16 ioprio;
108	__s32 fd;
109	union {
110	__u64 off;
111	__u64 addr2;
112	};
113	__u64 addr;
114	__u32 len;
115	union {
116	__kernel_rwf_t rw_flags;
117	__u32 fsync_flags;
118	__u16 poll_events;
119	__u32 sync_range_flags;
120	__u32 msg_flags;
121	__u32 timeout_flags;
122	__u32 accept_flags;
123	__u32 cancel_flags;
124	__u32 open_flags;
125	__u32 statx_flags;
126	};
127	__u64 user_data;
128	union {
129	__u16 buf_index;
130	__u64 __pad2[3];
131	};
132	};
133
134	struct io_uring_cqe
135	{
136	__u64 user_data;
137	__s32 res;
138	__u32 flags;
139	};
140
141	struct io_sqring_offsets
142	{
143	__u32 head;
144	__u32 tail;
145	__u32 ring_mask;
146	__u32 ring_entries;
147	__u32 flags;
148	__u32 dropped;
149	__u32 array;
150	__u32 resv1;
151	__u64 resv2;
152	};
153
154	struct io_cqring_offsets
155	{
156	__u32 head;
157	__u32 tail;
158	__u32 ring_mask;
159	__u32 ring_entries;
160	__u32 overflow;
161	__u32 cqes;
162	__u64 resv[2];
163	};
164
165	struct io_uring_params
166	{
167	__u32 sq_entries;
168	__u32 cq_entries;
169	__u32 flags;
170	__u32 sq_thread_cpu;
171	__u32 sq_thread_idle;
172	__u32 features;
173	__u32 resv[4];
174	struct io_sqring_offsets sq_off;
175	struct io_cqring_offsets cq_off;
176	};
177
178	#define IORING_OP_POLL_ADD 6
179	#define IORING_OP_POLL_REMOVE 7
180
181	#define IORING_ENTER_GETEVENTS 0x01
182
183	#define IORING_OFF_SQ_RING 0x00000000ULL
184	#define IORING_OFF_CQ_RING 0x08000000ULL
185	#define IORING_OFF_SQES 0x10000000ULL
186
187	#define IORING_FEAT_SINGLE_MMAP 0x1
188	#define IORING_FEAT_NODROP 0x2
189	#define IORING_FEAT_SUBMIT_STABLE 0x4
190
191	inline_size
192	int
193	evsys_io_uring_setup (unsigned entries, struct io_uring_params *params)
194	{
195	return ev_syscall2 (SYS_io_uring_setup, entries, params);
196	}
197
198	inline_size
199	int
200	evsys_io_uring_enter (int fd, unsigned to_submit, unsigned min_complete, unsigned flags, const sigset_t *sig, size_t sigsz)
201	{
202	return ev_syscall6 (SYS_io_uring_enter, fd, to_submit, min_complete, flags, sig, sigsz);
203	}
204
205	/*****************************************************************************/
206	/* actual backed implementation */
207
208	/* we hope that volatile will make the compiler access this variables only once */
209	#define EV_SQ_VAR(name) (volatile unsigned )((char *)iouring_sq_ring + iouring_sq_ ## name)
210	#define EV_CQ_VAR(name) (volatile unsigned )((char *)iouring_cq_ring + iouring_cq_ ## name)
211
212	/* the index array */
213	#define EV_SQ_ARRAY ((unsigned )((char )iouring_sq_ring + iouring_sq_array))
214
215	/* the submit/completion queue entries */
216	#define EV_SQES ((struct io_uring_sqe *) iouring_sqes)
217	#define EV_CQES ((struct io_uring_cqe )((char )iouring_cq_ring + iouring_cq_cqes))
218
219	static
220	struct io_uring_sqe *
221	iouring_sqe_get (EV_P)
222	{
223	unsigned tail = EV_SQ_VAR (tail);
224
225	if (tail + 1 - EV_SQ_VAR (head) > EV_SQ_VAR (ring_entries))
226	{
227	/* queue full, flush */
228	evsys_io_uring_enter (iouring_fd, iouring_to_submit, 0, 0, 0, 0);
229	iouring_to_submit = 0;
230	}
231
232	assert (("libev: io_uring queue full after flush", tail + 1 - EV_SQ_VAR (head) <= EV_SQ_VAR (ring_entries)));
233
234	return EV_SQES + (tail & EV_SQ_VAR (ring_mask));
235	}
236
237	inline_size
238	struct io_uring_sqe *
239	iouring_sqe_submit (EV_P_ struct io_uring_sqe *sqe)
240	{
241	unsigned idx = sqe - EV_SQES;
242
243	EV_SQ_ARRAY [idx] = idx;
244	ECB_MEMORY_FENCE_RELEASE;
245	++EV_SQ_VAR (tail);
246	/ECB_MEMORY_FENCE_RELEASE; / for the time being we assume this is not needed */
247	++iouring_to_submit;
248	}
249
250	/*****************************************************************************/
251
252	/* when the timerfd expires we simply note the fact,
253	* as the purpose of the timerfd is to wake us up, nothing else.
254	* the next iteration should re-set it.
255	*/
256	static void
257	iouring_tfd_cb (EV_P_ struct ev_io *w, int revents)
258	{
259	iouring_tfd_to = EV_TSTAMP_HUGE;
260	}
261
262	/* called for full and partial cleanup */
263	ecb_cold
264	static int
265	iouring_internal_destroy (EV_P)
266	{
267	close (iouring_tfd);
268	close (iouring_fd);
269
270	if (iouring_sq_ring != MAP_FAILED) munmap (iouring_sq_ring, iouring_sq_ring_size);
271	if (iouring_cq_ring != MAP_FAILED) munmap (iouring_cq_ring, iouring_cq_ring_size);
272	if (iouring_sqes != MAP_FAILED) munmap (iouring_sqes , iouring_sqes_size );
273
274	if (ev_is_active (&iouring_tfd_w))
275	{
276	ev_ref (EV_A);
277	ev_io_stop (EV_A_ &iouring_tfd_w);
278	}
279	}
280
281	ecb_cold
282	static int
283	iouring_internal_init (EV_P)
284	{
285	struct io_uring_params params = { 0 };
286
287	iouring_to_submit = 0;
288
289	iouring_tfd = -1;
290	iouring_sq_ring = MAP_FAILED;
291	iouring_cq_ring = MAP_FAILED;
292	iouring_sqes = MAP_FAILED;
293
294	for (;;)
295	{
296	iouring_fd = evsys_io_uring_setup (iouring_entries, &params);
297
298	if (iouring_fd >= 0)
299	break; /* yippie */
300
301	if (errno != EINVAL)
302	return -1; /* we failed */
303
304	#if TODO
305	if ((~params.features) & (IORING_FEAT_NODROP \| IORING_FEATURE_SINGLE_MMAP))
306	return -1; /* we require the above features */
307	#endif
308
309	/* EINVAL: lots of possible reasons, but maybe
310	* it is because we hit the unqueryable hardcoded size limit
311	*/
312
313	/* we hit the limit already, give up */
314	if (iouring_max_entries)
315	return -1;
316
317	/* first time we hit EINVAL? assume we hit the limit, so go back and retry */
318	iouring_entries >>= 1;
319	iouring_max_entries = iouring_entries;
320	}
321
322	iouring_sq_ring_size = params.sq_off.array + params.sq_entries * sizeof (unsigned);
323	iouring_cq_ring_size = params.cq_off.cqes + params.cq_entries * sizeof (struct io_uring_cqe);
324	iouring_sqes_size = params.sq_entries * sizeof (struct io_uring_sqe);
325
326	iouring_sq_ring = mmap (0, iouring_sq_ring_size, PROT_READ \| PROT_WRITE,
327	MAP_SHARED \| MAP_POPULATE, iouring_fd, IORING_OFF_SQ_RING);
328	iouring_cq_ring = mmap (0, iouring_cq_ring_size, PROT_READ \| PROT_WRITE,
329	MAP_SHARED \| MAP_POPULATE, iouring_fd, IORING_OFF_CQ_RING);
330	iouring_sqes = mmap (0, iouring_sqes_size, PROT_READ \| PROT_WRITE,
331	MAP_SHARED \| MAP_POPULATE, iouring_fd, IORING_OFF_SQES);
332
333	if (iouring_sq_ring == MAP_FAILED \|\| iouring_cq_ring == MAP_FAILED \|\| iouring_sqes == MAP_FAILED)
334	return -1;
335
336	iouring_sq_head = params.sq_off.head;
337	iouring_sq_tail = params.sq_off.tail;
338	iouring_sq_ring_mask = params.sq_off.ring_mask;
339	iouring_sq_ring_entries = params.sq_off.ring_entries;
340	iouring_sq_flags = params.sq_off.flags;
341	iouring_sq_dropped = params.sq_off.dropped;
342	iouring_sq_array = params.sq_off.array;
343
344	iouring_cq_head = params.cq_off.head;
345	iouring_cq_tail = params.cq_off.tail;
346	iouring_cq_ring_mask = params.cq_off.ring_mask;
347	iouring_cq_ring_entries = params.cq_off.ring_entries;
348	iouring_cq_overflow = params.cq_off.overflow;
349	iouring_cq_cqes = params.cq_off.cqes;
350
351	iouring_tfd = timerfd_create (CLOCK_MONOTONIC, TFD_CLOEXEC);
352
353	if (iouring_tfd < 0)
354	return iouring_tfd;
355
356	iouring_tfd_to = EV_TSTAMP_HUGE;
357
358	return 0;
359	}
360
361	ecb_cold
362	static void
363	iouring_fork (EV_P)
364	{
365	iouring_internal_destroy (EV_A);
366
367	while (iouring_internal_init (EV_A) < 0)
368	ev_syserr ("(libev) io_uring_setup");
369
370	fd_rearm_all (EV_A);
371
372	ev_io_stop (EV_A_ &iouring_tfd_w);
373	ev_io_set (EV_A_ &iouring_tfd_w, iouring_tfd, EV_READ);
374	ev_io_start (EV_A_ &iouring_tfd_w);
375	}
376
377	/*****************************************************************************/
378
379	static void
380	iouring_modify (EV_P_ int fd, int oev, int nev)
381	{
382	if (oev)
383	{
384	/* we assume the sqe's are all "properly" initialised */
385	struct io_uring_sqe *sqe = iouring_sqe_get (EV_A);
386	sqe->opcode = IORING_OP_POLL_REMOVE;
387	sqe->fd = fd;
388	sqe->user_data = -1;
389	iouring_sqe_submit (EV_A_ sqe);
390
391	/* increment generation counter to avoid handling old events */
392	++anfds [fd].egen;
393	}
394
395	if (nev)
396	{
397	struct io_uring_sqe *sqe = iouring_sqe_get (EV_A);
398	sqe->opcode = IORING_OP_POLL_ADD;
399	sqe->fd = fd;
400	sqe->user_data = (uint32_t)fd \| ((__u64)(uint32_t)anfds [fd].egen << 32);
401	sqe->poll_events =
402	(nev & EV_READ ? POLLIN : 0)
403	\| (nev & EV_WRITE ? POLLOUT : 0);
404	iouring_sqe_submit (EV_A_ sqe);
405	}
406	}
407
408	inline_size
409	void
410	iouring_tfd_update (EV_P_ ev_tstamp timeout)
411	{
412	ev_tstamp tfd_to = mn_now + timeout;
413
414	/* we assume there will be many iterations per timer change, so
415	* we only re-set the timerfd when we have to because its expiry
416	* is too late.
417	*/
418	if (ecb_expect_false (tfd_to < iouring_tfd_to))
419	{
420	struct itimerspec its;
421
422	iouring_tfd_to = tfd_to;
423	EV_TS_SET (its.it_interval, 0.);
424	EV_TS_SET (its.it_value, tfd_to);
425
426	if (timerfd_settime (iouring_tfd, TFD_TIMER_ABSTIME, &its, 0) < 0)
427	assert (("libev: iouring timerfd_settime failed", 0));
428	}
429	}
430
431	inline_size
432	void
433	iouring_process_cqe (EV_P_ struct io_uring_cqe *cqe)
434	{
435	int fd = cqe->user_data & 0xffffffffU;
436	uint32_t gen = cqe->user_data >> 32;
437	int res = cqe->res;
438
439	/* ignore fd removal events, if there are any. TODO: verify */
440	/* TODO: yes, this triggers */
441	if (cqe->user_data == (__u64)-1)
442	return;
443
444	assert (("libev: io_uring fd must be in-bounds", fd >= 0 && fd < anfdmax));
445
446	/* documentation lies, of course. the result value is NOT like
447	* normal syscalls, but like linux raw syscalls, i.e. negative
448	* error numbers. fortunate, as otherwise there would be no way
449	* to get error codes at all. still, why not document this?
450	*/
451
452	/* ignore event if generation doesn't match */
453	/* this should actually be very rare */
454	if (ecb_expect_false (gen != (uint32_t)anfds [fd].egen))
455	return;
456
457	if (ecb_expect_false (res < 0))
458	{
459	/TODO: EINVAL handling (was something failed with this fd)/
460	/TODO: EBUSY happens when?/
461
462	if (res == -EBADF)
463	{
464	assert (("libev: event loop rejected bad fd", res != -EBADF));
465	fd_kill (EV_A_ fd);
466	}
467	else
468	{
469	errno = -res;
470	ev_syserr ("(libev) IORING_OP_POLL_ADD");
471	}
472
473	return;
474	}
475
476	/* feed events, we do not expect or handle POLLNVAL */
477	fd_event (
478	EV_A_
479	fd,
480	(res & (POLLOUT \| POLLERR \| POLLHUP) ? EV_WRITE : 0)
481	\| (res & (POLLIN \| POLLERR \| POLLHUP) ? EV_READ : 0)
482	);
483
484	/* io_uring is oneshot, so we need to re-arm the fd next iteration */
485	/* this also means we usually have to do at least one syscall per iteration */
486	anfds [fd].events = 0;
487	fd_change (EV_A_ fd, EV_ANFD_REIFY);
488	}
489
490	/* called when the event queue overflows */
491	ecb_cold
492	static void
493	iouring_overflow (EV_P)
494	{
495	/* we have two options, resize the queue (by tearing down
496	* everything and recreating it, or living with it
497	* and polling.
498	* we implement this by resizing the queue, and, if that fails,
499	* we just recreate the state on every failure, which
500	* kind of is a very inefficient poll.
501	* one danger is, due to the bios toward lower fds,
502	* we will only really get events for those, so
503	* maybe we need a poll() fallback, after all.
504	*/
505	/EV_CQ_VAR (overflow) = 0;/ /* need to do this if we keep the state and poll manually */
506
507	fd_rearm_all (EV_A);
508
509	/* we double the size until we hit the hard-to-probe maximum */
510	if (!iouring_max_entries)
511	{
512	iouring_entries <<= 1;
513	iouring_fork (EV_A);
514	}
515	else
516	{
517	/* we hit the kernel limit, we should fall back to something else.
518	* we can either poll() a few times and hope for the best,
519	* poll always, or switch to epoll.
520	* TODO: is this necessary with newer kernels?
521	*/
522
523	iouring_internal_destroy (EV_A);
524
525	/* this should make it so that on return, we don't call any uring functions */
526	iouring_to_submit = 0;
527
528	for (;;)
529	{
530	backend = epoll_init (EV_A_ 0);
531
532	if (backend)
533	break;
534
535	ev_syserr ("(libev) iouring switch to epoll");
536	}
537	}
538	}
539
540	/* handle any events in the completion queue, return true if there were any */
541	static int
542	iouring_handle_cq (EV_P)
543	{
544	unsigned head, tail, mask;
545
546	head = EV_CQ_VAR (head);
547	ECB_MEMORY_FENCE_ACQUIRE;
548	tail = EV_CQ_VAR (tail);
549
550	if (head == tail)
551	return 0;
552
553	/* it can only overflow if we have events, yes, yes? */
554	if (ecb_expect_false (EV_CQ_VAR (overflow)))
555	{
556	iouring_overflow (EV_A);
557	return 1;
558	}
559
560	mask = EV_CQ_VAR (ring_mask);
561
562	do
563	iouring_process_cqe (EV_A_ &EV_CQES [head++ & mask]);
564	while (head != tail);
565
566	EV_CQ_VAR (head) = head;
567	ECB_MEMORY_FENCE_RELEASE;
568
569	return 1;
570	}
571
572	static void
573	iouring_poll (EV_P_ ev_tstamp timeout)
574	{
575	/* if we have events, no need for extra syscalls, but we might have to queue events */
576	if (iouring_handle_cq (EV_A))
577	timeout = EV_TS_CONST (0.);
578	else
579	/* no events, so maybe wait for some */
580	iouring_tfd_update (EV_A_ timeout);
581
582	/* only enter the kernel if we have something to submit, or we need to wait */
583	if (timeout \|\| iouring_to_submit)
584	{
585	int res;
586
587	EV_RELEASE_CB;
588
589	res = evsys_io_uring_enter (iouring_fd, iouring_to_submit, 1,
590	timeout > EV_TS_CONST (0.) ? IORING_ENTER_GETEVENTS : 0, 0, 0);
591	iouring_to_submit = 0;
592
593	EV_ACQUIRE_CB;
594
595	if (ecb_expect_false (res < 0))
596	if (errno == EINTR)
597	/* ignore */;
598	else
599	ev_syserr ("(libev) iouring setup");
600	else
601	iouring_handle_cq (EV_A);
602	}
603	}
604
605	inline_size
606	int
607	iouring_init (EV_P_ int flags)
608	{
609	iouring_entries = IOURING_INIT_ENTRIES;
610	iouring_max_entries = 0;
611
612	if (iouring_internal_init (EV_A) < 0)
613	{
614	iouring_internal_destroy (EV_A);
615	return 0;
616	}
617
618	ev_io_init (&iouring_tfd_w, iouring_tfd_cb, iouring_tfd, EV_READ);
619	ev_set_priority (&iouring_tfd_w, EV_MINPRI);
620	ev_io_start (EV_A_ &iouring_tfd_w);
621	ev_unref (EV_A); /* watcher should not keep loop alive */
622
623	backend_modify = iouring_modify;
624	backend_poll = iouring_poll;
625
626	return EVBACKEND_IOURING;
627	}
628
629	inline_size
630	void
631	iouring_destroy (EV_P)
632	{
633	iouring_internal_destroy (EV_A);
634	}
635