block/io_uring.c

   1 /*
   2  * Linux io_uring support.
   3  *
   4  * Copyright (C) 2009 IBM, Corp.
   5  * Copyright (C) 2009 Red Hat, Inc.
   6  * Copyright (C) 2019 Aarushi Mehta
   7  *
   8  * This work is licensed under the terms of the GNU GPL, version 2 or later.
   9  * See the COPYING file in the top-level directory.
  10  */
  11 #include "qemu/osdep.h"
  12 #include <liburing.h>
  13 #include "block/aio.h"
  14 #include "qemu/queue.h"
  15 #include "block/block.h"
  16 #include "block/raw-aio.h"
  17 #include "qemu/coroutine.h"
  18 #include "qapi/error.h"
  19 #include "trace.h"
  20
  21
  22 /* io_uring ring size */
  23 #define MAX_ENTRIES 128
  24
  25 typedef struct LuringAIOCB {
  26     Coroutine *co;
  27     struct io_uring_sqe sqeq;
  28     ssize_t ret;
  29     QEMUIOVector *qiov;
  30     bool is_read;
  31     QSIMPLEQ_ENTRY(LuringAIOCB) next;
  32
  33     /*
  34      * Buffered reads may require resubmission, see
  35      * luring_resubmit_short_read().
  36      */
  37     int total_read;
  38     QEMUIOVector resubmit_qiov;
  39 } LuringAIOCB;
  40
  41 typedef struct LuringQueue {
  42     int plugged;
  43     unsigned int in_queue;
  44     unsigned int in_flight;
  45     bool blocked;
  46     QSIMPLEQ_HEAD(, LuringAIOCB) submit_queue;
  47 } LuringQueue;
  48
  49 typedef struct LuringState {
  50     AioContext *aio_context;
  51
  52     struct io_uring ring;
  53
  54     /* io queue for submit at batch.  Protected by AioContext lock. */
  55     LuringQueue io_q;
  56
  57     /* I/O completion processing.  Only runs in I/O thread.  */
  58     QEMUBH *completion_bh;
  59 } LuringState;
  60
  61 /**
  62  * luring_resubmit:
  63  *
  64  * Resubmit a request by appending it to submit_queue.  The caller must ensure
  65  * that ioq_submit() is called later so that submit_queue requests are started.
  66  */
  67 static void luring_resubmit(LuringState *s, LuringAIOCB *luringcb)
  68 {
  69     QSIMPLEQ_INSERT_TAIL(&s->io_q.submit_queue, luringcb, next);
  70     s->io_q.in_queue++;
  71 }
  72
  73 /**
  74  * luring_resubmit_short_read:
  75  *
  76  * Before Linux commit 9d93a3f5a0c ("io_uring: punt short reads to async
  77  * context") a buffered I/O request with the start of the file range in the
  78  * page cache could result in a short read.  Applications need to resubmit the
  79  * remaining read request.
  80  *
  81  * This is a slow path but recent kernels never take it.
  82  */
  83 static void luring_resubmit_short_read(LuringState *s, LuringAIOCB *luringcb,
  84                                        int nread)
  85 {
  86     QEMUIOVector *resubmit_qiov;
  87     size_t remaining;
  88
  89     trace_luring_resubmit_short_read(s, luringcb, nread);
  90
  91     /* Update read position */
  92     luringcb->total_read = nread;
  93     remaining = luringcb->qiov->size - luringcb->total_read;
  94
  95     /* Shorten qiov */
  96     resubmit_qiov = &luringcb->resubmit_qiov;
  97     if (resubmit_qiov->iov == NULL) {
  98         qemu_iovec_init(resubmit_qiov, luringcb->qiov->niov);
  99     } else {
 100         qemu_iovec_reset(resubmit_qiov);
 101     }
 102     qemu_iovec_concat(resubmit_qiov, luringcb->qiov, luringcb->total_read,
 103                       remaining);
 104
 105     /* Update sqe */
 106     luringcb->sqeq.off = nread;
 107     luringcb->sqeq.addr = (__u64)(uintptr_t)luringcb->resubmit_qiov.iov;
 108     luringcb->sqeq.len = luringcb->resubmit_qiov.niov;
 109
 110     luring_resubmit(s, luringcb);
 111 }
 112
 113 /**
 114  * luring_process_completions:
 115  * @s: AIO state
 116  *
 117  * Fetches completed I/O requests, consumes cqes and invokes their callbacks
 118  * The function is somewhat tricky because it supports nested event loops, for
 119  * example when a request callback invokes aio_poll().
 120  *
 121  * Function schedules BH completion so it  can be called again in a nested
 122  * event loop.  When there are no events left  to complete the BH is being
 123  * canceled.
 124  *
 125  */
 126 static void luring_process_completions(LuringState *s)
 127 {
 128     struct io_uring_cqe *cqes;
 129     int total_bytes;
 130     /*
 131      * Request completion callbacks can run the nested event loop.
 132      * Schedule ourselves so the nested event loop will "see" remaining
 133      * completed requests and process them.  Without this, completion
 134      * callbacks that wait for other requests using a nested event loop
 135      * would hang forever.
 136      *
 137      * This workaround is needed because io_uring uses poll_wait, which
 138      * is woken up when new events are added to the uring, thus polling on
 139      * the same uring fd will block unless more events are received.
 140      *
 141      * Other leaf block drivers (drivers that access the data themselves)
 142      * are networking based, so they poll sockets for data and run the
 143      * correct coroutine.
 144      */
 145     qemu_bh_schedule(s->completion_bh);
 146
 147     while (io_uring_peek_cqe(&s->ring, &cqes) == 0) {
 148         LuringAIOCB *luringcb;
 149         int ret;
 150
 151         if (!cqes) {
 152             break;
 153         }
 154
 155         luringcb = io_uring_cqe_get_data(cqes);
 156         ret = cqes->res;
 157         io_uring_cqe_seen(&s->ring, cqes);
 158         cqes = NULL;
 159
 160         /* Change counters one-by-one because we can be nested. */
 161         s->io_q.in_flight--;
 162         trace_luring_process_completion(s, luringcb, ret);
 163
 164         /* total_read is non-zero only for resubmitted read requests */
 165         total_bytes = ret + luringcb->total_read;
 166
 167         if (ret < 0) {
 168             /*
 169              * Only writev/readv/fsync requests on regular files or host block
 170              * devices are submitted. Therefore -EAGAIN is not expected but it's
 171              * known to happen sometimes with Linux SCSI. Submit again and hope
 172              * the request completes successfully.
 173              *
 174              * For more information, see:
 175              * https://lore.kernel.org/io-uring/20210727165811.284510-3-axboe@kernel.dk/T/#u
 176              *
 177              * If the code is changed to submit other types of requests in the
 178              * future, then this workaround may need to be extended to deal with
 179              * genuine -EAGAIN results that should not be resubmitted
 180              * immediately.
 181              */
 182             if (ret == -EINTR || ret == -EAGAIN) {
 183                 luring_resubmit(s, luringcb);
 184                 continue;
 185             }
 186         } else if (!luringcb->qiov) {
 187             goto end;
 188         } else if (total_bytes == luringcb->qiov->size) {
 189             ret = 0;
 190         /* Only read/write */
 191         } else {
 192             /* Short Read/Write */
 193             if (luringcb->is_read) {
 194                 if (ret > 0) {
 195                     luring_resubmit_short_read(s, luringcb, ret);
 196                     continue;
 197                 } else {
 198                     /* Pad with zeroes */
 199                     qemu_iovec_memset(luringcb->qiov, total_bytes, 0,
 200                                       luringcb->qiov->size - total_bytes);
 201                     ret = 0;
 202                 }
 203             } else {
 204                 ret = -ENOSPC;
 205             }
 206         }
 207 end:
 208         luringcb->ret = ret;
 209         qemu_iovec_destroy(&luringcb->resubmit_qiov);
 210
 211         /*
 212          * If the coroutine is already entered it must be in ioq_submit()
 213          * and will notice luringcb->ret has been filled in when it
 214          * eventually runs later. Coroutines cannot be entered recursively
 215          * so avoid doing that!
 216          */
 217         if (!qemu_coroutine_entered(luringcb->co)) {
 218             aio_co_wake(luringcb->co);
 219         }
 220     }
 221     qemu_bh_cancel(s->completion_bh);
 222 }
 223
 224 static int ioq_submit(LuringState *s)
 225 {
 226     int ret = 0;
 227     LuringAIOCB *luringcb, *luringcb_next;
 228
 229     while (s->io_q.in_queue > 0) {
 230         /*
 231          * Try to fetch sqes from the ring for requests waiting in
 232          * the overflow queue
 233          */
 234         QSIMPLEQ_FOREACH_SAFE(luringcb, &s->io_q.submit_queue, next,
 235                               luringcb_next) {
 236             struct io_uring_sqe *sqes = io_uring_get_sqe(&s->ring);
 237             if (!sqes) {
 238                 break;
 239             }
 240             /* Prep sqe for submission */
 241             *sqes = luringcb->sqeq;
 242             QSIMPLEQ_REMOVE_HEAD(&s->io_q.submit_queue, next);
 243         }
 244         ret = io_uring_submit(&s->ring);
 245         trace_luring_io_uring_submit(s, ret);
 246         /* Prevent infinite loop if submission is refused */
 247         if (ret <= 0) {
 248             if (ret == -EAGAIN || ret == -EINTR) {
 249                 continue;
 250             }
 251             break;
 252         }
 253         s->io_q.in_flight += ret;
 254         s->io_q.in_queue  -= ret;
 255     }
 256     s->io_q.blocked = (s->io_q.in_queue > 0);
 257
 258     if (s->io_q.in_flight) {
 259         /*
 260          * We can try to complete something just right away if there are
 261          * still requests in-flight.
 262          */
 263         luring_process_completions(s);
 264     }
 265     return ret;
 266 }
 267
 268 static void luring_process_completions_and_submit(LuringState *s)
 269 {
 270     aio_context_acquire(s->aio_context);
 271     luring_process_completions(s);
 272
 273     if (!s->io_q.plugged && s->io_q.in_queue > 0) {
 274         ioq_submit(s);
 275     }
 276     aio_context_release(s->aio_context);
 277 }
 278
 279 static void qemu_luring_completion_bh(void *opaque)
 280 {
 281     LuringState *s = opaque;
 282     luring_process_completions_and_submit(s);
 283 }
 284
 285 static void qemu_luring_completion_cb(void *opaque)
 286 {
 287     LuringState *s = opaque;
 288     luring_process_completions_and_submit(s);
 289 }
 290
 291 static bool qemu_luring_poll_cb(void *opaque)
 292 {
 293     LuringState *s = opaque;
 294
 295     return io_uring_cq_ready(&s->ring);
 296 }
 297
 298 static void qemu_luring_poll_ready(void *opaque)
 299 {
 300     LuringState *s = opaque;
 301
 302     luring_process_completions_and_submit(s);
 303 }
 304
 305 static void ioq_init(LuringQueue *io_q)
 306 {
 307     QSIMPLEQ_INIT(&io_q->submit_queue);
 308     io_q->plugged = 0;
 309     io_q->in_queue = 0;
 310     io_q->in_flight = 0;
 311     io_q->blocked = false;
 312 }
 313
 314 void luring_io_plug(BlockDriverState *bs, LuringState *s)
 315 {
 316     trace_luring_io_plug(s);
 317     s->io_q.plugged++;
 318 }
 319
 320 void luring_io_unplug(BlockDriverState *bs, LuringState *s)
 321 {
 322     assert(s->io_q.plugged);
 323     trace_luring_io_unplug(s, s->io_q.blocked, s->io_q.plugged,
 324                            s->io_q.in_queue, s->io_q.in_flight);
 325     if (--s->io_q.plugged == 0 &&
 326         !s->io_q.blocked && s->io_q.in_queue > 0) {
 327         ioq_submit(s);
 328     }
 329 }
 330
 331 /**
 332  * luring_do_submit:
 333  * @fd: file descriptor for I/O
 334  * @luringcb: AIO control block
 335  * @s: AIO state
 336  * @offset: offset for request
 337  * @type: type of request
 338  *
 339  * Fetches sqes from ring, adds to pending queue and preps them
 340  *
 341  */
 342 static int luring_do_submit(int fd, LuringAIOCB *luringcb, LuringState *s,
 343                             uint64_t offset, int type)
 344 {
 345     int ret;
 346     struct io_uring_sqe *sqes = &luringcb->sqeq;
 347
 348     switch (type) {
 349     case QEMU_AIO_WRITE:
 350         io_uring_prep_writev(sqes, fd, luringcb->qiov->iov,
 351                              luringcb->qiov->niov, offset);
 352         break;
 353     case QEMU_AIO_READ:
 354         io_uring_prep_readv(sqes, fd, luringcb->qiov->iov,
 355                             luringcb->qiov->niov, offset);
 356         break;
 357     case QEMU_AIO_FLUSH:
 358         io_uring_prep_fsync(sqes, fd, IORING_FSYNC_DATASYNC);
 359         break;
 360     default:
 361         fprintf(stderr, "%s: invalid AIO request type, aborting 0x%x.\n",
 362                         __func__, type);
 363         abort();
 364     }
 365     io_uring_sqe_set_data(sqes, luringcb);
 366
 367     QSIMPLEQ_INSERT_TAIL(&s->io_q.submit_queue, luringcb, next);
 368     s->io_q.in_queue++;
 369     trace_luring_do_submit(s, s->io_q.blocked, s->io_q.plugged,
 370                            s->io_q.in_queue, s->io_q.in_flight);
 371     if (!s->io_q.blocked &&
 372         (!s->io_q.plugged ||
 373          s->io_q.in_flight + s->io_q.in_queue >= MAX_ENTRIES)) {
 374         ret = ioq_submit(s);
 375         trace_luring_do_submit_done(s, ret);
 376         return ret;
 377     }
 378     return 0;
 379 }
 380
 381 int coroutine_fn luring_co_submit(BlockDriverState *bs, LuringState *s, int fd,
 382                                   uint64_t offset, QEMUIOVector *qiov, int type)
 383 {
 384     int ret;
 385     LuringAIOCB luringcb = {
 386         .co         = qemu_coroutine_self(),
 387         .ret        = -EINPROGRESS,
 388         .qiov       = qiov,
 389         .is_read    = (type == QEMU_AIO_READ),
 390     };
 391     trace_luring_co_submit(bs, s, &luringcb, fd, offset, qiov ? qiov->size : 0,
 392                            type);
 393     ret = luring_do_submit(fd, &luringcb, s, offset, type);
 394
 395     if (ret < 0) {
 396         return ret;
 397     }
 398
 399     if (luringcb.ret == -EINPROGRESS) {
 400         qemu_coroutine_yield();
 401     }
 402     return luringcb.ret;
 403 }
 404
 405 void luring_detach_aio_context(LuringState *s, AioContext *old_context)
 406 {
 407     aio_set_fd_handler(old_context, s->ring.ring_fd, false,
 408                        NULL, NULL, NULL, NULL, s);
 409     qemu_bh_delete(s->completion_bh);
 410     s->aio_context = NULL;
 411 }
 412
 413 void luring_attach_aio_context(LuringState *s, AioContext *new_context)
 414 {
 415     s->aio_context = new_context;
 416     s->completion_bh = aio_bh_new(new_context, qemu_luring_completion_bh, s);
 417     aio_set_fd_handler(s->aio_context, s->ring.ring_fd, false,
 418                        qemu_luring_completion_cb, NULL,
 419                        qemu_luring_poll_cb, qemu_luring_poll_ready, s);
 420 }
 421
 422 LuringState *luring_init(Error **errp)
 423 {
 424     int rc;
 425     LuringState *s = g_new0(LuringState, 1);
 426     struct io_uring *ring = &s->ring;
 427
 428     trace_luring_init_state(s, sizeof(*s));
 429
 430     rc = io_uring_queue_init(MAX_ENTRIES, ring, 0);
 431     if (rc < 0) {
 432         error_setg_errno(errp, errno, "failed to init linux io_uring ring");
 433         g_free(s);
 434         return NULL;
 435     }
 436
 437     ioq_init(&s->io_q);
 438 #ifdef CONFIG_LIBURING_REGISTER_RING_FD
 439     if (io_uring_register_ring_fd(&s->ring) < 0) {
 440         /*
 441          * Only warn about this error: we will fallback to the non-optimized
 442          * io_uring operations.
 443          */
 444         warn_report("failed to register linux io_uring ring file descriptor");
 445     }
 446 #endif
 447
 448     return s;
 449 }
 450
 451 void luring_cleanup(LuringState *s)
 452 {
 453     io_uring_queue_exit(&s->ring);
 454     trace_luring_cleanup_state(s);
 455     g_free(s);
 456 }