futex: Support Windows

Windows supports futex-like APIs since Windows 8 and Windows Server
2012.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Link: https://lore.kernel.org/r/20250529-event-v5-2-53b285203794@daynix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/futex.h

@@ -1,5 +1,5 @@
 /*
- * Wrappers around Linux futex syscall
+ * Wrappers around Linux futex syscall and similar
  *
  * Copyright Red Hat, Inc. 2017
  *
@@ -11,28 +11,37 @@
  *
  */
 
+/*
+ * Note that a wake-up can also be caused by common futex usage patterns in
+ * unrelated code that happened to have previously used the futex word's
+ * memory location (e.g., typical futex-based implementations of Pthreads
+ * mutexes can cause this under some conditions).  Therefore, qemu_futex_wait()
+ * callers should always conservatively assume that it is a spurious wake-up,
+ * and use the futex word's value (i.e., the user-space synchronization scheme)
+ * to decide whether to continue to block or not.
+ */
+
 #ifndef QEMU_FUTEX_H
 #define QEMU_FUTEX_H
 
+#define HAVE_FUTEX
+
+#ifdef CONFIG_LINUX
 #include <sys/syscall.h>
 #include <linux/futex.h>
 
 #define qemu_futex(...)              syscall(__NR_futex, __VA_ARGS__)
 
-static inline void qemu_futex_wake(void *f, int n)
+static inline void qemu_futex_wake_all(void *f)
 {
-    qemu_futex(f, FUTEX_WAKE, n, NULL, NULL, 0);
+    qemu_futex(f, FUTEX_WAKE, INT_MAX, NULL, NULL, 0);
+}
+
+static inline void qemu_futex_wake_single(void *f)
+{
+    qemu_futex(f, FUTEX_WAKE, 1, NULL, NULL, 0);
 }
 
-/*
- * Note that a wake-up can also be caused by common futex usage patterns in
- * unrelated code that happened to have previously used the futex word's
- * memory location (e.g., typical futex-based implementations of Pthreads
- * mutexes can cause this under some conditions).  Therefore, callers should
- * always conservatively assume that it is a spurious wake-up, and use the futex
- * word's value (i.e., the user-space synchronization scheme) to decide whether
- * to continue to block or not.
- */
 static inline void qemu_futex_wait(void *f, unsigned val)
 {
     while (qemu_futex(f, FUTEX_WAIT, (int) val, NULL, NULL, 0)) {
@@ -46,5 +55,25 @@ static inline void qemu_futex_wait(void *f, unsigned val)
         }
     }
 }
+#elif defined(CONFIG_WIN32)
+#include <synchapi.h>
+
+static inline void qemu_futex_wake_all(void *f)
+{
+    WakeByAddressAll(f);
+}
+
+static inline void qemu_futex_wake_single(void *f)
+{
+    WakeByAddressSingle(f);
+}
+
+static inline void qemu_futex_wait(void *f, unsigned val)
+{
+    WaitOnAddress(f, &val, sizeof(val), INFINITE);
+}
+#else
+#undef HAVE_FUTEX
+#endif
 
 #endif /* QEMU_FUTEX_H */

meson.build

@@ -838,11 +838,18 @@ emulator_link_args = []
 midl = not_found
 widl = not_found
 pathcch = not_found
+synchronization = not_found
 host_dsosuf = '.so'
 if host_os == 'windows'
   midl = find_program('midl', required: false)
   widl = find_program('widl', required: false)
   pathcch = cc.find_library('pathcch')
+  synchronization = cc.find_library('Synchronization', required: false)
+  if not synchronization.found()
+    # The library name is lowercase on mingw
+    synchronization = cc.find_library('synchronization', required: true)
+  endif
+
   socket = cc.find_library('ws2_32')
   winmm = cc.find_library('winmm')
 

tests/unit/test-aio-multithread.c

@@ -330,7 +330,7 @@ static void mcs_mutex_unlock(void)
     /* Wake up the next in line.  */
     next = qatomic_read(&nodes[id].next);
     nodes[next].locked = 0;
-    qemu_futex_wake(&nodes[next].locked, 1);
+    qemu_futex_wake_single(&nodes[next].locked);
 }
 
 static void test_multi_fair_mutex_entry(void *opaque)

util/lockcnt.c

@@ -106,7 +106,7 @@ static bool qemu_lockcnt_cmpxchg_or_wait(QemuLockCnt *lockcnt, int *val,
 static void lockcnt_wake(QemuLockCnt *lockcnt)
 {
     trace_lockcnt_futex_wake(lockcnt);
-    qemu_futex_wake(&lockcnt->count, 1);
+    qemu_futex_wake_single(&lockcnt->count);
 }
 
 void qemu_lockcnt_inc(QemuLockCnt *lockcnt)

util/meson.build

@@ -27,7 +27,7 @@ else
   util_ss.add(files('event_notifier-win32.c'))
   util_ss.add(files('oslib-win32.c'))
   util_ss.add(files('qemu-thread-win32.c'))
-  util_ss.add(winmm, pathcch)
+  util_ss.add(winmm, pathcch, synchronization)
 endif
 util_ss.add(when: linux_io_uring, if_true: files('fdmon-io_uring.c'))
 if glib_has_gslice

util/qemu-thread-posix.c

@@ -345,7 +345,7 @@ static inline void qemu_futex_wait(QemuEvent *ev, unsigned val)
 
 /* Valid transitions:
  * - free->set, when setting the event
- * - busy->set, when setting the event, followed by qemu_futex_wake
+ * - busy->set, when setting the event, followed by qemu_futex_wake_all
  * - set->free, when resetting the event
  * - free->busy, when waiting
  *
@@ -400,7 +400,7 @@ void qemu_event_set(QemuEvent *ev)
         smp_mb__after_rmw();
         if (old == EV_BUSY) {
             /* There were waiters, wake them up.  */
-            qemu_futex_wake(ev, INT_MAX);
+            qemu_futex_wake_all(ev);
         }
     }
 }