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qemu-thread: Use futex for QemuEvent on Windows

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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>
This commit is contained in:
Akihiko Odaki 2025-05-26 14:29:13 +09:00 • committed by Paolo Bonzini
parent d1895f4c17
commit 69e10db83e
7 changed files with 182 additions and 315 deletions
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129
util/qemu-thread-win32.c
View file

@ -231,135 +231,6 @@ void qemu_sem_wait(QemuSemaphore *sem)
}
}
/* Wrap a Win32 manual-reset event with a fast userspace path. The idea
* is to reset the Win32 event lazily, as part of a test-reset-test-wait
* sequence. Such a sequence is, indeed, how QemuEvents are used by
* RCU and other subsystems!
*
* Valid transitions:
* - free->set, when setting the event
* - busy->set, when setting the event, followed by SetEvent
* - set->free, when resetting the event
* - free->busy, when waiting
*
* 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 (and is faster than cmpxchg).
*/
#define EV_SET 0
#define EV_FREE 1
#define EV_BUSY -1
void qemu_event_init(QemuEvent *ev, bool init)
{
/* Manual reset. */
ev->event = CreateEvent(NULL, TRUE, TRUE, NULL);
ev->value = (init ? EV_SET : EV_FREE);
ev->initialized = true;
}
void qemu_event_destroy(QemuEvent *ev)
{
assert(ev->initialized);
ev->initialized = false;
CloseHandle(ev->event);
}
void qemu_event_set(QemuEvent *ev)
{
assert(ev->initialized);
/*
* 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 after ResetEvent. */
smp_mb__after_rmw();
if (old == EV_BUSY) {
/* There were waiters, wake them up. */
SetEvent(ev->event);
}
}
}
void qemu_event_reset(QemuEvent *ev)
{
assert(ev->initialized);
/*
* 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();
}
void qemu_event_wait(QemuEvent *ev)
{
unsigned value;
assert(ev->initialized);
/*
* 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().
*
* If we do go down the slow path, there is no requirement at all: we
* might miss a qemu_event_set() here but ultimately the memory barrier in
* qemu_futex_wait() will ensure the check is done correctly.
*/
value = qatomic_load_acquire(&ev->value);
if (value != EV_SET) {
if (value == EV_FREE) {
/*
* Here the underlying kernel event is reset, but qemu_event_set is
* not yet going to call SetEvent. However, there will be another
* check for EV_SET below when setting EV_BUSY. At that point it
* is safe to call WaitForSingleObject.
*/
ResetEvent(ev->event);
/*
* It is not clear whether ResetEvent provides this barrier; kernel
* APIs (KeResetEvent/KeClearEvent) do not. Better safe than sorry!
*/
smp_mb();
/*
* 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.
*/
if (qatomic_cmpxchg(&ev->value, EV_FREE, EV_BUSY) == EV_SET) {
return;
}
}
/*
* ev->value is now EV_BUSY. Since we didn't observe EV_SET,
* qemu_event_set() must observe EV_BUSY and call SetEvent().
*/
WaitForSingleObject(ev->event, INFINITE);
}
}
struct QemuThreadData {
/* Passed to win32_start_routine. */
void *(*start_routine)(void *);