qemu

fdmon-io_uring.c (11476B)

---

 /* SPDX-License-Identifier: GPL-2.0-or-later */
 /*
  * Linux io_uring file descriptor monitoring
  *
  * The Linux io_uring API supports file descriptor monitoring with a few
  * advantages over existing APIs like poll(2) and epoll(7):
  *
  * 1. Userspace polling of events is possible because the completion queue (cq
  *    ring) is shared between the kernel and userspace.  This allows
  *    applications that rely on userspace polling to also monitor file
  *    descriptors in the same userspace polling loop.
  *
  * 2. Submission and completion is batched and done together in a single system
  *    call.  This minimizes the number of system calls.
  *
  * 3. File descriptor monitoring is O(1) like epoll(7) so it scales better than
  *    poll(2).
  *
  * 4. Nanosecond timeouts are supported so it requires fewer syscalls than
  *    epoll(7).
  *
  * This code only monitors file descriptors and does not do asynchronous disk
  * I/O.  Implementing disk I/O efficiently has other requirements and should
  * use a separate io_uring so it does not make sense to unify the code.
  *
  * File descriptor monitoring is implemented using the following operations:
  *
  * 1. IORING_OP_POLL_ADD - adds a file descriptor to be monitored.
  * 2. IORING_OP_POLL_REMOVE - removes a file descriptor being monitored.  When
  *    the poll mask changes for a file descriptor it is first removed and then
  *    re-added with the new poll mask, so this operation is also used as part
  *    of modifying an existing monitored file descriptor.
  * 3. IORING_OP_TIMEOUT - added every time a blocking syscall is made to wait
  *    for events.  This operation self-cancels if another event completes
  *    before the timeout.
  *
  * io_uring calls the submission queue the "sq ring" and the completion queue
  * the "cq ring".  Ring entries are called "sqe" and "cqe", respectively.
  *
  * The code is structured so that sq/cq rings are only modified within
  * fdmon_io_uring_wait().  Changes to AioHandlers are made by enqueuing them on
  * ctx->submit_list so that fdmon_io_uring_wait() can submit IORING_OP_POLL_ADD
  * and/or IORING_OP_POLL_REMOVE sqes for them.
  */
 
 #include "qemu/osdep.h"
 #include <poll.h>
 #include "qemu/rcu_queue.h"
 #include "aio-posix.h"
 
 enum {
     FDMON_IO_URING_ENTRIES  = 128, /* sq/cq ring size */
 
     /* AioHandler::flags */
     FDMON_IO_URING_PENDING  = (1 << 0),
     FDMON_IO_URING_ADD      = (1 << 1),
     FDMON_IO_URING_REMOVE   = (1 << 2),
 };
 
 static inline int poll_events_from_pfd(int pfd_events)
 {
     return (pfd_events & G_IO_IN ? POLLIN : 0) |
            (pfd_events & G_IO_OUT ? POLLOUT : 0) |
            (pfd_events & G_IO_HUP ? POLLHUP : 0) |
            (pfd_events & G_IO_ERR ? POLLERR : 0);
 }
 
 static inline int pfd_events_from_poll(int poll_events)
 {
     return (poll_events & POLLIN ? G_IO_IN : 0) |
            (poll_events & POLLOUT ? G_IO_OUT : 0) |
            (poll_events & POLLHUP ? G_IO_HUP : 0) |
            (poll_events & POLLERR ? G_IO_ERR : 0);
 }
 
 /*
  * Returns an sqe for submitting a request.  Only be called within
  * fdmon_io_uring_wait().
  */
 static struct io_uring_sqe *get_sqe(AioContext *ctx)
 {
     struct io_uring *ring = &ctx->fdmon_io_uring;
     struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
     int ret;
 
     if (likely(sqe)) {
         return sqe;
     }
 
     /* No free sqes left, submit pending sqes first */
     do {
         ret = io_uring_submit(ring);
     } while (ret == -EINTR);
 
     assert(ret > 1);
     sqe = io_uring_get_sqe(ring);
     assert(sqe);
     return sqe;
 }
 
 /* Atomically enqueue an AioHandler for sq ring submission */
 static void enqueue(AioHandlerSList *head, AioHandler *node, unsigned flags)
 {
     unsigned old_flags;
 
     old_flags = qatomic_fetch_or(&node->flags, FDMON_IO_URING_PENDING | flags);
     if (!(old_flags & FDMON_IO_URING_PENDING)) {
         QSLIST_INSERT_HEAD_ATOMIC(head, node, node_submitted);
     }
 }
 
 /* Dequeue an AioHandler for sq ring submission.  Called by fill_sq_ring(). */
 static AioHandler *dequeue(AioHandlerSList *head, unsigned *flags)
 {
     AioHandler *node = QSLIST_FIRST(head);
 
     if (!node) {
         return NULL;
     }
 
     /* Doesn't need to be atomic since fill_sq_ring() moves the list */
     QSLIST_REMOVE_HEAD(head, node_submitted);
 
     /*
      * Don't clear FDMON_IO_URING_REMOVE.  It's sticky so it can serve two
      * purposes: telling fill_sq_ring() to submit IORING_OP_POLL_REMOVE and
      * telling process_cqe() to delete the AioHandler when its
      * IORING_OP_POLL_ADD completes.
      */
     *flags = qatomic_fetch_and(&node->flags, ~(FDMON_IO_URING_PENDING |
                                               FDMON_IO_URING_ADD));
     return node;
 }
 
 static void fdmon_io_uring_update(AioContext *ctx,
                                   AioHandler *old_node,
                                   AioHandler *new_node)
 {
     if (new_node) {
         enqueue(&ctx->submit_list, new_node, FDMON_IO_URING_ADD);
     }
 
     if (old_node) {
         /*
          * Deletion is tricky because IORING_OP_POLL_ADD and
          * IORING_OP_POLL_REMOVE are async.  We need to wait for the original
          * IORING_OP_POLL_ADD to complete before this handler can be freed
          * safely.
          *
          * It's possible that the file descriptor becomes ready and the
          * IORING_OP_POLL_ADD cqe is enqueued before IORING_OP_POLL_REMOVE is
          * submitted, too.
          *
          * Mark this handler deleted right now but don't place it on
          * ctx->deleted_aio_handlers yet.  Instead, manually fudge the list
          * entry to make QLIST_IS_INSERTED() think this handler has been
          * inserted and other code recognizes this AioHandler as deleted.
          *
          * Once the original IORING_OP_POLL_ADD completes we enqueue the
          * handler on the real ctx->deleted_aio_handlers list to be freed.
          */
         assert(!QLIST_IS_INSERTED(old_node, node_deleted));
         old_node->node_deleted.le_prev = &old_node->node_deleted.le_next;
 
         enqueue(&ctx->submit_list, old_node, FDMON_IO_URING_REMOVE);
     }
 }
 
 static void add_poll_add_sqe(AioContext *ctx, AioHandler *node)
 {
     struct io_uring_sqe *sqe = get_sqe(ctx);
     int events = poll_events_from_pfd(node->pfd.events);
 
     io_uring_prep_poll_add(sqe, node->pfd.fd, events);
     io_uring_sqe_set_data(sqe, node);
 }
 
 static void add_poll_remove_sqe(AioContext *ctx, AioHandler *node)
 {
     struct io_uring_sqe *sqe = get_sqe(ctx);
 
 #ifdef LIBURING_HAVE_DATA64
     io_uring_prep_poll_remove(sqe, (__u64)(uintptr_t)node);
 #else
     io_uring_prep_poll_remove(sqe, node);
 #endif
 }
 
 /* Add a timeout that self-cancels when another cqe becomes ready */
 static void add_timeout_sqe(AioContext *ctx, int64_t ns)
 {
     struct io_uring_sqe *sqe;
     struct __kernel_timespec ts = {
         .tv_sec = ns / NANOSECONDS_PER_SECOND,
         .tv_nsec = ns % NANOSECONDS_PER_SECOND,
     };
 
     sqe = get_sqe(ctx);
     io_uring_prep_timeout(sqe, &ts, 1, 0);
 }
 
 /* Add sqes from ctx->submit_list for submission */
 static void fill_sq_ring(AioContext *ctx)
 {
     AioHandlerSList submit_list;
     AioHandler *node;
     unsigned flags;
 
     QSLIST_MOVE_ATOMIC(&submit_list, &ctx->submit_list);
 
     while ((node = dequeue(&submit_list, &flags))) {
         /* Order matters, just in case both flags were set */
         if (flags & FDMON_IO_URING_ADD) {
             add_poll_add_sqe(ctx, node);
         }
         if (flags & FDMON_IO_URING_REMOVE) {
             add_poll_remove_sqe(ctx, node);
         }
     }
 }
 
 /* Returns true if a handler became ready */
 static bool process_cqe(AioContext *ctx,
                         AioHandlerList *ready_list,
                         struct io_uring_cqe *cqe)
 {
     AioHandler *node = io_uring_cqe_get_data(cqe);
     unsigned flags;
 
     /* poll_timeout and poll_remove have a zero user_data field */
     if (!node) {
         return false;
     }
 
     /*
      * Deletion can only happen when IORING_OP_POLL_ADD completes.  If we race
      * with enqueue() here then we can safely clear the FDMON_IO_URING_REMOVE
      * bit before IORING_OP_POLL_REMOVE is submitted.
      */
     flags = qatomic_fetch_and(&node->flags, ~FDMON_IO_URING_REMOVE);
     if (flags & FDMON_IO_URING_REMOVE) {
         QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_deleted);
         return false;
     }
 
     aio_add_ready_handler(ready_list, node, pfd_events_from_poll(cqe->res));
 
     /* IORING_OP_POLL_ADD is one-shot so we must re-arm it */
     add_poll_add_sqe(ctx, node);
     return true;
 }
 
 static int process_cq_ring(AioContext *ctx, AioHandlerList *ready_list)
 {
     struct io_uring *ring = &ctx->fdmon_io_uring;
     struct io_uring_cqe *cqe;
     unsigned num_cqes = 0;
     unsigned num_ready = 0;
     unsigned head;
 
     io_uring_for_each_cqe(ring, head, cqe) {
         if (process_cqe(ctx, ready_list, cqe)) {
             num_ready++;
         }
 
         num_cqes++;
     }
 
     io_uring_cq_advance(ring, num_cqes);
     return num_ready;
 }
 
 static int fdmon_io_uring_wait(AioContext *ctx, AioHandlerList *ready_list,
                                int64_t timeout)
 {
     unsigned wait_nr = 1; /* block until at least one cqe is ready */
     int ret;
 
     /* Fall back while external clients are disabled */
     if (qatomic_read(&ctx->external_disable_cnt)) {
         return fdmon_poll_ops.wait(ctx, ready_list, timeout);
     }
 
     if (timeout == 0) {
         wait_nr = 0; /* non-blocking */
     } else if (timeout > 0) {
         add_timeout_sqe(ctx, timeout);
     }
 
     fill_sq_ring(ctx);
 
     do {
         ret = io_uring_submit_and_wait(&ctx->fdmon_io_uring, wait_nr);
     } while (ret == -EINTR);
 
     assert(ret >= 0);
 
     return process_cq_ring(ctx, ready_list);
 }
 
 static bool fdmon_io_uring_need_wait(AioContext *ctx)
 {
     /* Have io_uring events completed? */
     if (io_uring_cq_ready(&ctx->fdmon_io_uring)) {
         return true;
     }
 
     /* Are there pending sqes to submit? */
     if (io_uring_sq_ready(&ctx->fdmon_io_uring)) {
         return true;
     }
 
     /* Do we need to process AioHandlers for io_uring changes? */
     if (!QSLIST_EMPTY_RCU(&ctx->submit_list)) {
         return true;
     }
 
     /* Are we falling back to fdmon-poll? */
     return qatomic_read(&ctx->external_disable_cnt);
 }
 
 static const FDMonOps fdmon_io_uring_ops = {
     .update = fdmon_io_uring_update,
     .wait = fdmon_io_uring_wait,
     .need_wait = fdmon_io_uring_need_wait,
 };
 
 bool fdmon_io_uring_setup(AioContext *ctx)
 {
     int ret;
 
     ret = io_uring_queue_init(FDMON_IO_URING_ENTRIES, &ctx->fdmon_io_uring, 0);
     if (ret != 0) {
         return false;
     }
 
     QSLIST_INIT(&ctx->submit_list);
     ctx->fdmon_ops = &fdmon_io_uring_ops;
     return true;
 }
 
 void fdmon_io_uring_destroy(AioContext *ctx)
 {
     if (ctx->fdmon_ops == &fdmon_io_uring_ops) {
         AioHandler *node;
 
         io_uring_queue_exit(&ctx->fdmon_io_uring);
 
         /* Move handlers due to be removed onto the deleted list */
         while ((node = QSLIST_FIRST_RCU(&ctx->submit_list))) {
             unsigned flags = qatomic_fetch_and(&node->flags,
                     ~(FDMON_IO_URING_PENDING |
                       FDMON_IO_URING_ADD |
                       FDMON_IO_URING_REMOVE));
 
             if (flags & FDMON_IO_URING_REMOVE) {
                 QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_deleted);
             }
 
             QSLIST_REMOVE_HEAD_RCU(&ctx->submit_list, node_submitted);
         }
 
         ctx->fdmon_ops = &fdmon_poll_ops;
     }
 }