util: Add coroutine backend for emscripten

Emscripten does not support couroutine methods currently used by QEMU but
provides a coroutine implementation called "fiber". This commit introduces a
coroutine backend using fiber. Note that fiber does not support submitting
coroutines to other threads.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/006b683fd578ed6303a2dc8679094da9a7e6dfb4.1745820062.git.ktokunaga.mail@gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/coroutine-wasm.c

@@ -0,0 +1,127 @@
+/*
+ * emscripten fiber coroutine initialization code
+ * based on coroutine-ucontext.c
+ *
+ * Copyright (C) 2006  Anthony Liguori <anthony@codemonkey.ws>
+ * Copyright (C) 2011  Kevin Wolf <kwolf@redhat.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.0 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/coroutine_int.h"
+#include "qemu/coroutine-tls.h"
+
+#include <emscripten/fiber.h>
+
+typedef struct {
+    Coroutine base;
+    void *stack;
+    size_t stack_size;
+
+    void *asyncify_stack;
+    size_t asyncify_stack_size;
+
+    CoroutineAction action;
+
+    emscripten_fiber_t fiber;
+} CoroutineEmscripten;
+
+/**
+ * Per-thread coroutine bookkeeping
+ */
+QEMU_DEFINE_STATIC_CO_TLS(Coroutine *, current);
+QEMU_DEFINE_STATIC_CO_TLS(CoroutineEmscripten *, leader);
+size_t leader_asyncify_stack_size = COROUTINE_STACK_SIZE;
+
+static void coroutine_trampoline(void *co_)
+{
+    Coroutine *co = co_;
+
+    while (true) {
+        co->entry(co->entry_arg);
+        qemu_coroutine_switch(co, co->caller, COROUTINE_TERMINATE);
+    }
+}
+
+Coroutine *qemu_coroutine_new(void)
+{
+    CoroutineEmscripten *co;
+
+    co = g_malloc0(sizeof(*co));
+
+    co->stack_size = COROUTINE_STACK_SIZE;
+    co->stack = qemu_alloc_stack(&co->stack_size);
+
+    co->asyncify_stack_size = COROUTINE_STACK_SIZE;
+    co->asyncify_stack = g_malloc0(co->asyncify_stack_size);
+    emscripten_fiber_init(&co->fiber, coroutine_trampoline, &co->base,
+                          co->stack, co->stack_size, co->asyncify_stack,
+                          co->asyncify_stack_size);
+
+    return &co->base;
+}
+
+void qemu_coroutine_delete(Coroutine *co_)
+{
+    CoroutineEmscripten *co = DO_UPCAST(CoroutineEmscripten, base, co_);
+
+    qemu_free_stack(co->stack, co->stack_size);
+    g_free(co->asyncify_stack);
+    g_free(co);
+}
+
+CoroutineAction qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
+                      CoroutineAction action)
+{
+    CoroutineEmscripten *from = DO_UPCAST(CoroutineEmscripten, base, from_);
+    CoroutineEmscripten *to = DO_UPCAST(CoroutineEmscripten, base, to_);
+
+    set_current(to_);
+    to->action = action;
+    emscripten_fiber_swap(&from->fiber, &to->fiber);
+    return from->action;
+}
+
+Coroutine *qemu_coroutine_self(void)
+{
+    Coroutine *self = get_current();
+
+    if (!self) {
+        CoroutineEmscripten *leaderp = get_leader();
+        if (!leaderp) {
+            leaderp = g_malloc0(sizeof(*leaderp));
+            leaderp->asyncify_stack = g_malloc0(leader_asyncify_stack_size);
+            leaderp->asyncify_stack_size = leader_asyncify_stack_size;
+            emscripten_fiber_init_from_current_context(
+                &leaderp->fiber,
+                leaderp->asyncify_stack,
+                leaderp->asyncify_stack_size);
+            leaderp->stack = leaderp->fiber.stack_limit;
+            leaderp->stack_size =
+                leaderp->fiber.stack_base - leaderp->fiber.stack_limit;
+            set_leader(leaderp);
+        }
+        self = &leaderp->base;
+        set_current(self);
+    }
+    return self;
+}
+
+bool qemu_in_coroutine(void)
+{
+    Coroutine *self = get_current();
+
+    return self && self->caller;
+}