Merge remote-tracking branch 'remotes/pmaydell/tags/pull-target-arm-20210330' into...
[qemu/ar7.git] / util / fdmon-io_uring.c
blob1461dfa40743ba0a33dcb3e7e795a43265f4ff3d
1 /* SPDX-License-Identifier: GPL-2.0-or-later */
2 /*
3 * Linux io_uring file descriptor monitoring
5 * The Linux io_uring API supports file descriptor monitoring with a few
6 * advantages over existing APIs like poll(2) and epoll(7):
8 * 1. Userspace polling of events is possible because the completion queue (cq
9 * ring) is shared between the kernel and userspace. This allows
10 * applications that rely on userspace polling to also monitor file
11 * descriptors in the same userspace polling loop.
13 * 2. Submission and completion is batched and done together in a single system
14 * call. This minimizes the number of system calls.
16 * 3. File descriptor monitoring is O(1) like epoll(7) so it scales better than
17 * poll(2).
19 * 4. Nanosecond timeouts are supported so it requires fewer syscalls than
20 * epoll(7).
22 * This code only monitors file descriptors and does not do asynchronous disk
23 * I/O. Implementing disk I/O efficiently has other requirements and should
24 * use a separate io_uring so it does not make sense to unify the code.
26 * File descriptor monitoring is implemented using the following operations:
28 * 1. IORING_OP_POLL_ADD - adds a file descriptor to be monitored.
29 * 2. IORING_OP_POLL_REMOVE - removes a file descriptor being monitored. When
30 * the poll mask changes for a file descriptor it is first removed and then
31 * re-added with the new poll mask, so this operation is also used as part
32 * of modifying an existing monitored file descriptor.
33 * 3. IORING_OP_TIMEOUT - added every time a blocking syscall is made to wait
34 * for events. This operation self-cancels if another event completes
35 * before the timeout.
37 * io_uring calls the submission queue the "sq ring" and the completion queue
38 * the "cq ring". Ring entries are called "sqe" and "cqe", respectively.
40 * The code is structured so that sq/cq rings are only modified within
41 * fdmon_io_uring_wait(). Changes to AioHandlers are made by enqueuing them on
42 * ctx->submit_list so that fdmon_io_uring_wait() can submit IORING_OP_POLL_ADD
43 * and/or IORING_OP_POLL_REMOVE sqes for them.
46 #include "qemu/osdep.h"
47 #include <poll.h>
48 #include "qemu/rcu_queue.h"
49 #include "aio-posix.h"
51 enum {
52 FDMON_IO_URING_ENTRIES = 128, /* sq/cq ring size */
54 /* AioHandler::flags */
55 FDMON_IO_URING_PENDING = (1 << 0),
56 FDMON_IO_URING_ADD = (1 << 1),
57 FDMON_IO_URING_REMOVE = (1 << 2),
60 static inline int poll_events_from_pfd(int pfd_events)
62 return (pfd_events & G_IO_IN ? POLLIN : 0) |
63 (pfd_events & G_IO_OUT ? POLLOUT : 0) |
64 (pfd_events & G_IO_HUP ? POLLHUP : 0) |
65 (pfd_events & G_IO_ERR ? POLLERR : 0);
68 static inline int pfd_events_from_poll(int poll_events)
70 return (poll_events & POLLIN ? G_IO_IN : 0) |
71 (poll_events & POLLOUT ? G_IO_OUT : 0) |
72 (poll_events & POLLHUP ? G_IO_HUP : 0) |
73 (poll_events & POLLERR ? G_IO_ERR : 0);
77 * Returns an sqe for submitting a request. Only be called within
78 * fdmon_io_uring_wait().
80 static struct io_uring_sqe *get_sqe(AioContext *ctx)
82 struct io_uring *ring = &ctx->fdmon_io_uring;
83 struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
84 int ret;
86 if (likely(sqe)) {
87 return sqe;
90 /* No free sqes left, submit pending sqes first */
91 do {
92 ret = io_uring_submit(ring);
93 } while (ret == -EINTR);
95 assert(ret > 1);
96 sqe = io_uring_get_sqe(ring);
97 assert(sqe);
98 return sqe;
101 /* Atomically enqueue an AioHandler for sq ring submission */
102 static void enqueue(AioHandlerSList *head, AioHandler *node, unsigned flags)
104 unsigned old_flags;
106 old_flags = qatomic_fetch_or(&node->flags, FDMON_IO_URING_PENDING | flags);
107 if (!(old_flags & FDMON_IO_URING_PENDING)) {
108 QSLIST_INSERT_HEAD_ATOMIC(head, node, node_submitted);
112 /* Dequeue an AioHandler for sq ring submission. Called by fill_sq_ring(). */
113 static AioHandler *dequeue(AioHandlerSList *head, unsigned *flags)
115 AioHandler *node = QSLIST_FIRST(head);
117 if (!node) {
118 return NULL;
121 /* Doesn't need to be atomic since fill_sq_ring() moves the list */
122 QSLIST_REMOVE_HEAD(head, node_submitted);
125 * Don't clear FDMON_IO_URING_REMOVE. It's sticky so it can serve two
126 * purposes: telling fill_sq_ring() to submit IORING_OP_POLL_REMOVE and
127 * telling process_cqe() to delete the AioHandler when its
128 * IORING_OP_POLL_ADD completes.
130 *flags = qatomic_fetch_and(&node->flags, ~(FDMON_IO_URING_PENDING |
131 FDMON_IO_URING_ADD));
132 return node;
135 static void fdmon_io_uring_update(AioContext *ctx,
136 AioHandler *old_node,
137 AioHandler *new_node)
139 if (new_node) {
140 enqueue(&ctx->submit_list, new_node, FDMON_IO_URING_ADD);
143 if (old_node) {
145 * Deletion is tricky because IORING_OP_POLL_ADD and
146 * IORING_OP_POLL_REMOVE are async. We need to wait for the original
147 * IORING_OP_POLL_ADD to complete before this handler can be freed
148 * safely.
150 * It's possible that the file descriptor becomes ready and the
151 * IORING_OP_POLL_ADD cqe is enqueued before IORING_OP_POLL_REMOVE is
152 * submitted, too.
154 * Mark this handler deleted right now but don't place it on
155 * ctx->deleted_aio_handlers yet. Instead, manually fudge the list
156 * entry to make QLIST_IS_INSERTED() think this handler has been
157 * inserted and other code recognizes this AioHandler as deleted.
159 * Once the original IORING_OP_POLL_ADD completes we enqueue the
160 * handler on the real ctx->deleted_aio_handlers list to be freed.
162 assert(!QLIST_IS_INSERTED(old_node, node_deleted));
163 old_node->node_deleted.le_prev = &old_node->node_deleted.le_next;
165 enqueue(&ctx->submit_list, old_node, FDMON_IO_URING_REMOVE);
169 static void add_poll_add_sqe(AioContext *ctx, AioHandler *node)
171 struct io_uring_sqe *sqe = get_sqe(ctx);
172 int events = poll_events_from_pfd(node->pfd.events);
174 io_uring_prep_poll_add(sqe, node->pfd.fd, events);
175 io_uring_sqe_set_data(sqe, node);
178 static void add_poll_remove_sqe(AioContext *ctx, AioHandler *node)
180 struct io_uring_sqe *sqe = get_sqe(ctx);
182 io_uring_prep_poll_remove(sqe, node);
185 /* Add a timeout that self-cancels when another cqe becomes ready */
186 static void add_timeout_sqe(AioContext *ctx, int64_t ns)
188 struct io_uring_sqe *sqe;
189 struct __kernel_timespec ts = {
190 .tv_sec = ns / NANOSECONDS_PER_SECOND,
191 .tv_nsec = ns % NANOSECONDS_PER_SECOND,
194 sqe = get_sqe(ctx);
195 io_uring_prep_timeout(sqe, &ts, 1, 0);
198 /* Add sqes from ctx->submit_list for submission */
199 static void fill_sq_ring(AioContext *ctx)
201 AioHandlerSList submit_list;
202 AioHandler *node;
203 unsigned flags;
205 QSLIST_MOVE_ATOMIC(&submit_list, &ctx->submit_list);
207 while ((node = dequeue(&submit_list, &flags))) {
208 /* Order matters, just in case both flags were set */
209 if (flags & FDMON_IO_URING_ADD) {
210 add_poll_add_sqe(ctx, node);
212 if (flags & FDMON_IO_URING_REMOVE) {
213 add_poll_remove_sqe(ctx, node);
218 /* Returns true if a handler became ready */
219 static bool process_cqe(AioContext *ctx,
220 AioHandlerList *ready_list,
221 struct io_uring_cqe *cqe)
223 AioHandler *node = io_uring_cqe_get_data(cqe);
224 unsigned flags;
226 /* poll_timeout and poll_remove have a zero user_data field */
227 if (!node) {
228 return false;
232 * Deletion can only happen when IORING_OP_POLL_ADD completes. If we race
233 * with enqueue() here then we can safely clear the FDMON_IO_URING_REMOVE
234 * bit before IORING_OP_POLL_REMOVE is submitted.
236 flags = qatomic_fetch_and(&node->flags, ~FDMON_IO_URING_REMOVE);
237 if (flags & FDMON_IO_URING_REMOVE) {
238 QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_deleted);
239 return false;
242 aio_add_ready_handler(ready_list, node, pfd_events_from_poll(cqe->res));
244 /* IORING_OP_POLL_ADD is one-shot so we must re-arm it */
245 add_poll_add_sqe(ctx, node);
246 return true;
249 static int process_cq_ring(AioContext *ctx, AioHandlerList *ready_list)
251 struct io_uring *ring = &ctx->fdmon_io_uring;
252 struct io_uring_cqe *cqe;
253 unsigned num_cqes = 0;
254 unsigned num_ready = 0;
255 unsigned head;
257 io_uring_for_each_cqe(ring, head, cqe) {
258 if (process_cqe(ctx, ready_list, cqe)) {
259 num_ready++;
262 num_cqes++;
265 io_uring_cq_advance(ring, num_cqes);
266 return num_ready;
269 static int fdmon_io_uring_wait(AioContext *ctx, AioHandlerList *ready_list,
270 int64_t timeout)
272 unsigned wait_nr = 1; /* block until at least one cqe is ready */
273 int ret;
275 /* Fall back while external clients are disabled */
276 if (qatomic_read(&ctx->external_disable_cnt)) {
277 return fdmon_poll_ops.wait(ctx, ready_list, timeout);
280 if (timeout == 0) {
281 wait_nr = 0; /* non-blocking */
282 } else if (timeout > 0) {
283 add_timeout_sqe(ctx, timeout);
286 fill_sq_ring(ctx);
288 do {
289 ret = io_uring_submit_and_wait(&ctx->fdmon_io_uring, wait_nr);
290 } while (ret == -EINTR);
292 assert(ret >= 0);
294 return process_cq_ring(ctx, ready_list);
297 static bool fdmon_io_uring_need_wait(AioContext *ctx)
299 /* Have io_uring events completed? */
300 if (io_uring_cq_ready(&ctx->fdmon_io_uring)) {
301 return true;
304 /* Are there pending sqes to submit? */
305 if (io_uring_sq_ready(&ctx->fdmon_io_uring)) {
306 return true;
309 /* Do we need to process AioHandlers for io_uring changes? */
310 if (!QSLIST_EMPTY_RCU(&ctx->submit_list)) {
311 return true;
314 /* Are we falling back to fdmon-poll? */
315 return qatomic_read(&ctx->external_disable_cnt);
318 static const FDMonOps fdmon_io_uring_ops = {
319 .update = fdmon_io_uring_update,
320 .wait = fdmon_io_uring_wait,
321 .need_wait = fdmon_io_uring_need_wait,
324 bool fdmon_io_uring_setup(AioContext *ctx)
326 int ret;
328 ret = io_uring_queue_init(FDMON_IO_URING_ENTRIES, &ctx->fdmon_io_uring, 0);
329 if (ret != 0) {
330 return false;
333 QSLIST_INIT(&ctx->submit_list);
334 ctx->fdmon_ops = &fdmon_io_uring_ops;
335 return true;
338 void fdmon_io_uring_destroy(AioContext *ctx)
340 if (ctx->fdmon_ops == &fdmon_io_uring_ops) {
341 AioHandler *node;
343 io_uring_queue_exit(&ctx->fdmon_io_uring);
345 /* Move handlers due to be removed onto the deleted list */
346 while ((node = QSLIST_FIRST_RCU(&ctx->submit_list))) {
347 unsigned flags = qatomic_fetch_and(&node->flags,
348 ~(FDMON_IO_URING_PENDING |
349 FDMON_IO_URING_ADD |
350 FDMON_IO_URING_REMOVE));
352 if (flags & FDMON_IO_URING_REMOVE) {
353 QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_deleted);
356 QSLIST_REMOVE_HEAD_RCU(&ctx->submit_list, node_submitted);
359 ctx->fdmon_ops = &fdmon_poll_ops;