[Qemu-devel] [PATCH v2 1/4] coroutine: introduce coroutines

[Stefan Hajnoczi]

From: Kevin Wolf <address@hidden>

Asynchronous code is becoming very complex.  At the same time
synchronous code is growing because it is convenient to write.
Sometimes duplicate code paths are even added, one synchronous and the
other asynchronous.  This patch introduces coroutines which allow code
that looks synchronous but is asynchronous under the covers.

A coroutine has its own stack and is therefore able to preserve state
across blocking operations, which traditionally require callback
functions and manual marshalling of parameters.

Creating and starting a coroutine is easy:

  coroutine = qemu_coroutine_create(my_coroutine);
  qemu_coroutine_enter(coroutine, my_data);

The coroutine then executes until it returns or yields:

  void coroutine_fn my_coroutine(void *opaque) {
      MyData *my_data = opaque;

      /* do some work */

      qemu_coroutine_yield();

      /* do some more work */
  }

Yielding switches control back to the caller of qemu_coroutine_enter().
This is typically used to switch back to the main thread's event loop
after issuing an asynchronous I/O request.  The request callback will
then invoke qemu_coroutine_enter() once more to switch back to the
coroutine.

Note that coroutines never execute concurrently and should only be used
from threads which hold the global mutex.  This restriction makes
programming with coroutines easier than with threads.  Race conditions
cannot occur since only one coroutine may be active at any time.  Other
coroutines can only run across yield.

This coroutines implementation is based on the gtk-vnc implementation
written by Anthony Liguori <address@hidden> but it has been
significantly rewritten by Kevin Wolf <address@hidden> to use
setjmp()/longjmp() instead of the more expensive swapcontext().

Signed-off-by: Kevin Wolf <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 Makefile.objs        |    7 +++
 coroutine-ucontext.c |   73 +++++++++++++++++++++++++++++
 coroutine-win32.c    |   57 +++++++++++++++++++++++
 qemu-coroutine-int.h |   53 +++++++++++++++++++++
 qemu-coroutine.c     |  125 ++++++++++++++++++++++++++++++++++++++++++++++++++
 qemu-coroutine.h     |   82 +++++++++++++++++++++++++++++++++
 trace-events         |    5 ++
 7 files changed, 402 insertions(+), 0 deletions(-)
 create mode 100644 coroutine-ucontext.c
 create mode 100644 coroutine-win32.c
 create mode 100644 qemu-coroutine-int.h
 create mode 100644 qemu-coroutine.c
 create mode 100644 qemu-coroutine.h

diff --git a/Makefile.objs b/Makefile.objs
index 9d8851e..cba6c2b 100644
--- a/Makefile.objs
+++ b/Makefile.objs
@@ -11,6 +11,12 @@ oslib-obj-$(CONFIG_WIN32) += oslib-win32.o
 oslib-obj-$(CONFIG_POSIX) += oslib-posix.o
 
 #######################################################################
+# coroutines
+coroutine-obj-y = qemu-coroutine.o
+coroutine-obj-$(CONFIG_POSIX) += coroutine-ucontext.o
+coroutine-obj-$(CONFIG_WIN32) += coroutine-win32.o
+
+#######################################################################
 # block-obj-y is code used by both qemu system emulation and qemu-img
 
 block-obj-y = cutils.o cache-utils.o qemu-malloc.o qemu-option.o module.o 
async.o
@@ -67,6 +73,7 @@ common-obj-y += readline.o console.o cursor.o qemu-error.o
 common-obj-y += $(oslib-obj-y)
 common-obj-$(CONFIG_WIN32) += os-win32.o
 common-obj-$(CONFIG_POSIX) += os-posix.o
+common-obj-y += $(coroutine-obj-y)
 
 common-obj-y += tcg-runtime.o host-utils.o
 common-obj-y += irq.o ioport.o input.o
diff --git a/coroutine-ucontext.c b/coroutine-ucontext.c
new file mode 100644
index 0000000..3b14ebf
--- /dev/null
+++ b/coroutine-ucontext.c
@@ -0,0 +1,73 @@
+/*
+ * ucontext coroutine initialization code
+ *
+ * Copyright (C) 2006  Anthony Liguori <address@hidden>
+ * Copyright (C) 2011  Kevin Wolf <address@hidden>
+ *
+ * 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, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301 
USA
+ */
+
+/* XXX Is there a nicer way to disable glibc's stack check for longjmp? */
+#ifdef _FORTIFY_SOURCE
+#undef _FORTIFY_SOURCE
+#endif
+#include <setjmp.h>
+#include <stdint.h>
+#include <ucontext.h>
+#include "qemu-coroutine-int.h"
+
+static Coroutine *new_coroutine;
+
+static void continuation_trampoline(void)
+{
+    Coroutine *co = new_coroutine;
+
+    /* Initialize longjmp environment and switch back to
+     * qemu_coroutine_init_env() in the old ucontext. */
+    if (!setjmp(co->env)) {
+        return;
+    }
+
+    while (true) {
+        co->entry(co->data);
+        if (!setjmp(co->env)) {
+            longjmp(co->caller->env, COROUTINE_TERMINATE);
+        }
+    }
+}
+
+int qemu_coroutine_init_env(Coroutine *co, size_t stack_size)
+{
+    ucontext_t old_uc, uc;
+
+    /* Create a new ucontext for switching to the coroutine stack and setting
+     * up a longjmp environment. */
+    if (getcontext(&uc) == -1) {
+        return -errno;
+    }
+
+    uc.uc_link = &old_uc;
+    uc.uc_stack.ss_sp = co->stack;
+    uc.uc_stack.ss_size = stack_size;
+    uc.uc_stack.ss_flags = 0;
+
+    new_coroutine = co;
+    makecontext(&uc, (void *)continuation_trampoline, 0);
+
+    /* Initialize the longjmp environment */
+    swapcontext(&old_uc, &uc);
+
+    return 0;
+}
diff --git a/coroutine-win32.c b/coroutine-win32.c
new file mode 100644
index 0000000..99141dd
--- /dev/null
+++ b/coroutine-win32.c
@@ -0,0 +1,57 @@
+/*
+ * Win32 coroutine initialization code
+ *
+ * Copyright (c) 2011 Kevin Wolf <address@hidden>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to 
deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "qemu-coroutine-int.h"
+
+static void __attribute__((used)) trampoline(Coroutine *co)
+{
+    if (!setjmp(co->env)) {
+        return;
+    }
+
+    while (true) {
+        co->entry(co->data);
+        if (!setjmp(co->env)) {
+            longjmp(co->caller->env, COROUTINE_TERMINATE);
+        }
+    }
+}
+
+int qemu_coroutine_init_env(Coroutine *co, size_t stack_size)
+{
+#ifdef __i386__
+    asm volatile(
+        "mov %%esp, %%ebx;"
+        "mov %0, %%esp;"
+        "pushl %1;"
+        "call _trampoline;"
+        "mov %%ebx, %%esp;"
+        : : "r" (co->stack + stack_size), "r" (co) : "ebx"
+    );
+#else
+    #error This host architecture is not supported for win32
+#endif
+
+    return 0;
+}
diff --git a/qemu-coroutine-int.h b/qemu-coroutine-int.h
new file mode 100644
index 0000000..71c6ee9
--- /dev/null
+++ b/qemu-coroutine-int.h
@@ -0,0 +1,53 @@
+/*
+ * Coroutine internals
+ *
+ * Copyright (c) 2011 Kevin Wolf <address@hidden>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to 
deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#ifndef QEMU_COROUTINE_INT_H
+#define QEMU_COROUTINE_INT_H
+
+#include <setjmp.h>
+#include "qemu-common.h"
+#include "qemu-queue.h"
+#include "qemu-coroutine.h"
+
+enum {
+    /* setjmp() return values */
+    COROUTINE_YIELD = 1,
+    COROUTINE_TERMINATE = 2,
+};
+
+struct Coroutine {
+    struct Coroutine *caller;
+
+    char *stack;
+
+    /* Used to pass arguments/return values for coroutines */
+    void *data;
+    CoroutineEntry *entry;
+
+    jmp_buf env;
+};
+
+int qemu_coroutine_init_env(Coroutine *co, size_t stack_size);
+
+#endif
diff --git a/qemu-coroutine.c b/qemu-coroutine.c
new file mode 100644
index 0000000..80bed14
--- /dev/null
+++ b/qemu-coroutine.c
@@ -0,0 +1,125 @@
+/*
+ * QEMU coroutines
+ *
+ * Copyright IBM, Corp. 2011
+ *
+ * Authors:
+ *  Stefan Hajnoczi    <address@hidden>
+ *  Kevin Wolf         <address@hidden>
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+
+/* XXX Is there a nicer way to disable glibc's stack check for longjmp? */
+#ifdef _FORTIFY_SOURCE
+#undef _FORTIFY_SOURCE
+#endif
+#include <setjmp.h>
+
+#include "trace.h"
+#include "qemu-queue.h"
+#include "qemu-common.h"
+#include "qemu-coroutine.h"
+#include "qemu-coroutine-int.h"
+
+static __thread Coroutine leader;
+static __thread Coroutine *current;
+
+static void coroutine_terminate(Coroutine *co)
+{
+    trace_qemu_coroutine_terminate(co);
+    qemu_free(co->stack);
+    qemu_free(co);
+}
+
+static Coroutine *coroutine_new(void)
+{
+    const size_t stack_size = 4 << 20;
+    Coroutine *co;
+
+    co = qemu_mallocz(sizeof(*co));
+    co->stack = qemu_malloc(stack_size);
+    qemu_coroutine_init_env(co, stack_size);
+    return co;
+}
+
+Coroutine *qemu_coroutine_create(CoroutineEntry *entry)
+{
+    Coroutine *co;
+
+    co = coroutine_new();
+    co->entry = entry;
+
+    return co;
+}
+
+Coroutine *coroutine_fn qemu_coroutine_self(void)
+{
+    if (current == NULL) {
+        current = &leader;
+    }
+
+    return current;
+}
+
+bool qemu_in_coroutine(void)
+{
+    return qemu_coroutine_self() != &leader;
+}
+
+static void *coroutine_swap(Coroutine *from, Coroutine *to, void *opaque)
+{
+    int ret;
+    void *to_data;
+
+    to->data = opaque;
+
+    ret = setjmp(from->env);
+    switch (ret) {
+    case COROUTINE_YIELD:
+        return from->data;
+    case COROUTINE_TERMINATE:
+        current = to->caller;
+        to_data = to->data;
+        coroutine_terminate(to);
+        return to_data;
+    default:
+        /* Switch to called coroutine */
+        current = to;
+        longjmp(to->env, COROUTINE_YIELD);
+        return NULL;
+    }
+}
+
+void qemu_coroutine_enter(Coroutine *co, void *opaque)
+{
+    Coroutine *self = qemu_coroutine_self();
+
+    trace_qemu_coroutine_enter(self, co, opaque);
+
+    if (co->caller) {
+        fprintf(stderr, "Co-routine re-entered recursively\n");
+        abort();
+    }
+
+    co->caller = self;
+    coroutine_swap(self, co, opaque);
+}
+
+void *coroutine_fn qemu_coroutine_yield(void)
+{
+    Coroutine *self = qemu_coroutine_self();
+    Coroutine *to = self->caller;
+
+    trace_qemu_coroutine_yield(self, self->caller);
+
+    if (!to) {
+        fprintf(stderr, "Co-routine is yielding to no one\n");
+        abort();
+    }
+
+    self->caller = NULL;
+    return coroutine_swap(self, to, NULL);
+}
diff --git a/qemu-coroutine.h b/qemu-coroutine.h
new file mode 100644
index 0000000..b79b4bf
--- /dev/null
+++ b/qemu-coroutine.h
@@ -0,0 +1,82 @@
+/*
+ * QEMU coroutine implementation
+ *
+ * Copyright IBM, Corp. 2011
+ *
+ * Authors:
+ *  Stefan Hajnoczi    <address@hidden>
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+
+#ifndef QEMU_COROUTINE_H
+#define QEMU_COROUTINE_H
+
+#include <stdbool.h>
+
+/**
+ * Mark a function that executes in coroutine context
+ *
+ * Functions that execute in coroutine context cannot be called directly from
+ * normal functions.  In the future it would be nice to enable compiler or
+ * static checker support for catching such errors.  This annotation might make
+ * it possible and in the meantime it serves as documentation.
+ *
+ * For example:
+ *
+ *   static void coroutine_fn foo(void) {
+ *       ....
+ *   }
+ */
+#define coroutine_fn
+
+typedef struct Coroutine Coroutine;
+
+/**
+ * Coroutine entry point
+ *
+ * When the coroutine is entered for the first time, opaque is passed in as an
+ * argument.
+ *
+ * When this function returns, the coroutine is destroyed automatically and
+ * execution continues in the caller who last entered the coroutine.
+ */
+typedef void coroutine_fn CoroutineEntry(void *opaque);
+
+/**
+ * Create a new coroutine
+ *
+ * Use qemu_coroutine_enter() to actually transfer control to the coroutine.
+ */
+Coroutine *qemu_coroutine_create(CoroutineEntry *entry);
+
+/**
+ * Transfer control to a coroutine
+ *
+ * The opaque argument is made available to the coroutine either as the entry
+ * function argument if this is the first time a new coroutine is entered, or
+ * as the return value from qemu_coroutine_yield().
+ */
+void qemu_coroutine_enter(Coroutine *coroutine, void *opaque);
+
+/**
+ * Transfer control back to a coroutine's caller
+ *
+ * The return value is the argument passed back in from the next
+ * qemu_coroutine_enter().
+ */
+void *coroutine_fn qemu_coroutine_yield(void);
+
+/**
+ * Get the currently executing coroutine
+ */
+Coroutine *coroutine_fn qemu_coroutine_self(void);
+
+/**
+ * Return whether or not currently inside a coroutine
+ */
+bool qemu_in_coroutine(void);
+
+#endif /* QEMU_COROUTINE_H */
diff --git a/trace-events b/trace-events
index 4f965e2..2d4db05 100644
--- a/trace-events
+++ b/trace-events
@@ -361,3 +361,8 @@ disable milkymist_uart_pulse_irq_tx(void) "Pulse IRQ TX"
 # hw/milkymist-vgafb.c
 disable milkymist_vgafb_memory_read(uint32_t addr, uint32_t value) "addr %08x 
value %08x"
 disable milkymist_vgafb_memory_write(uint32_t addr, uint32_t value) "addr %08x 
value %08x"
+
+# qemu-coroutine.c
+qemu_coroutine_enter(void *from, void *to, void *opaque) "from %p to %p opaque 
%p"
+qemu_coroutine_yield(void *from, void *to) "from %p to %p"
+qemu_coroutine_terminate(void *co) "self %p"
-- 
1.7.4.4