qemu-thread: Use futex for QemuEvent on Windows

Use the futex-based implementation of QemuEvent on Windows to
remove code duplication and remove the overhead of event object
construction and destruction.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20250526-event-v4-6-5b784cc8e1de@daynix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 171 insertions, 0 deletions

diff --git a/util/event.c b/util/event.c
new file mode 100644
index 0000000000..5a8141cd0e
--- /dev/null
+++ b/util/event.c
@@ -0,0 +1,171 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include "qemu/osdep.h"
+#include "qemu/thread.h"
+
+/*
+ * Valid transitions:
+ * - FREE -> SET (qemu_event_set)
+ * - BUSY -> SET (qemu_event_set)
+ * - SET -> FREE (qemu_event_reset)
+ * - FREE -> BUSY (qemu_event_wait)
+ *
+ * With futex, the waking and blocking operations follow
+ * BUSY -> SET and FREE -> BUSY, respectively.
+ *
+ * Without futex, BUSY -> SET and FREE -> BUSY never happen. Instead, the waking
+ * operation follows FREE -> SET and the blocking operation will happen in
+ * qemu_event_wait() if the event is not SET.
+ *
+ * SET->BUSY does not happen (it can be observed from the outside but
+ * it really is SET->FREE->BUSY).
+ *
+ * busy->free provably cannot happen; to enforce it, the set->free transition
+ * is done with an OR, which becomes a no-op if the event has concurrently
+ * transitioned to free or busy.
+ */
+
+#define EV_SET         0
+#define EV_FREE        1
+#define EV_BUSY       -1
+
+void qemu_event_init(QemuEvent *ev, bool init)
+{
+#ifndef HAVE_FUTEX
+    pthread_mutex_init(&ev->lock, NULL);
+    pthread_cond_init(&ev->cond, NULL);
+#endif
+
+    ev->value = (init ? EV_SET : EV_FREE);
+    ev->initialized = true;
+}
+
+void qemu_event_destroy(QemuEvent *ev)
+{
+    assert(ev->initialized);
+    ev->initialized = false;
+#ifndef HAVE_FUTEX
+    pthread_mutex_destroy(&ev->lock);
+    pthread_cond_destroy(&ev->cond);
+#endif
+}
+
+void qemu_event_set(QemuEvent *ev)
+{
+    assert(ev->initialized);
+
+#ifdef HAVE_FUTEX
+    /*
+     * Pairs with both qemu_event_reset() and qemu_event_wait().
+     *
+     * qemu_event_set has release semantics, but because it *loads*
+     * ev->value we need a full memory barrier here.
+     */
+    smp_mb();
+    if (qatomic_read(&ev->value) != EV_SET) {
+        int old = qatomic_xchg(&ev->value, EV_SET);
+
+        /* Pairs with memory barrier in kernel futex_wait system call.  */
+        smp_mb__after_rmw();
+        if (old == EV_BUSY) {
+            /* There were waiters, wake them up.  */
+            qemu_futex_wake_all(ev);
+        }
+    }
+#else
+    pthread_mutex_lock(&ev->lock);
+    /* Pairs with qemu_event_reset()'s load acquire.  */
+    qatomic_store_release(&ev->value, EV_SET);
+    pthread_cond_broadcast(&ev->cond);
+    pthread_mutex_unlock(&ev->lock);
+#endif
+}
+
+void qemu_event_reset(QemuEvent *ev)
+{
+    assert(ev->initialized);
+
+#ifdef HAVE_FUTEX
+    /*
+     * If there was a concurrent reset (or even reset+wait),
+     * do nothing.  Otherwise change EV_SET->EV_FREE.
+     */
+    qatomic_or(&ev->value, EV_FREE);
+
+    /*
+     * Order reset before checking the condition in the caller.
+     * Pairs with the first memory barrier in qemu_event_set().
+     */
+    smp_mb__after_rmw();
+#else
+    /*
+     * If futexes are not available, there are no EV_FREE->EV_BUSY
+     * transitions because wakeups are done entirely through the
+     * condition variable.  Since qatomic_set() only writes EV_FREE,
+     * the load seems useless but in reality, the acquire synchronizes
+     * with qemu_event_set()'s store release: if qemu_event_reset()
+     * sees EV_SET here, then the caller will certainly see a
+     * successful condition and skip qemu_event_wait():
+     *
+     * done = 1;                 if (done == 0)
+     * qemu_event_set() {          qemu_event_reset() {
+     *   lock();
+     *   ev->value = EV_SET ----->     load ev->value
+     *                                 ev->value = old value | EV_FREE
+     *   cond_broadcast()
+     *   unlock();                 }
+     * }                           if (done == 0)
+     *                               // qemu_event_wait() not called
+     */
+    qatomic_set(&ev->value, qatomic_load_acquire(&ev->value) | EV_FREE);
+#endif
+}
+
+void qemu_event_wait(QemuEvent *ev)
+{
+    assert(ev->initialized);
+
+#ifdef HAVE_FUTEX
+    while (true) {
+        /*
+         * qemu_event_wait must synchronize with qemu_event_set even if it does
+         * not go down the slow path, so this load-acquire is needed that
+         * synchronizes with the first memory barrier in qemu_event_set().
+         */
+        unsigned value = qatomic_load_acquire(&ev->value);
+        if (value == EV_SET) {
+            break;
+        }
+
+        if (value == EV_FREE) {
+            /*
+             * Leave the event reset and tell qemu_event_set that there are
+             * waiters.  No need to retry, because there cannot be a concurrent
+             * busy->free transition.  After the CAS, the event will be either
+             * set or busy.
+             *
+             * This cmpxchg doesn't have particular ordering requirements if it
+             * succeeds (moving the store earlier can only cause
+             * qemu_event_set() to issue _more_ wakeups), the failing case needs
+             * acquire semantics like the load above.
+             */
+            if (qatomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) {
+                break;
+            }
+        }
+
+        /*
+         * This is the final check for a concurrent set, so it does need
+         * a smp_mb() pairing with the second barrier of qemu_event_set().
+         * The barrier is inside the FUTEX_WAIT system call.
+         */
+        qemu_futex_wait(ev, EV_BUSY);
+    }
+#else
+    pthread_mutex_lock(&ev->lock);
+    while (qatomic_read(&ev->value) != EV_SET) {
+        pthread_cond_wait(&ev->cond, &ev->lock);
+    }
+    pthread_mutex_unlock(&ev->lock);
+#endif
+}