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, 0 insertions, 170 deletions

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 7fafbedbc4..ba725444ba 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -317,176 +317,6 @@ void qemu_sem_wait(QemuSemaphore *sem)
     qemu_mutex_unlock(&sem->mutex);
 }
 
-#ifdef CONFIG_LINUX
-#include "qemu/futex.h"
-#endif
-
-/* 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 CONFIG_LINUX
-    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 CONFIG_LINUX
-    pthread_mutex_destroy(&ev->lock);
-    pthread_cond_destroy(&ev->cond);
-#endif
-}
-
-void qemu_event_set(QemuEvent *ev)
-{
-    assert(ev->initialized);
-
-#ifdef CONFIG_LINUX
-    /*
-     * 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 CONFIG_LINUX
-    /*
-     * 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 CONFIG_LINUX
-    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
-}
-
 static __thread NotifierList thread_exit;
 
 /*