io: follow coroutine AioContext in qio_channel_yield()

The ongoing QEMU multi-queue block layer effort makes it possible for multiple
threads to process I/O in parallel. The nbd block driver is not compatible with
the multi-queue block layer yet because QIOChannel cannot be used easily from
coroutines running in multiple threads. This series changes the QIOChannel API
to make that possible.

In the current API, calling qio_channel_attach_aio_context() sets the
AioContext where qio_channel_yield() installs an fd handler prior to yielding:

  qio_channel_attach_aio_context(ioc, my_ctx);
  ...
  qio_channel_yield(ioc); // my_ctx is used here
  ...
  qio_channel_detach_aio_context(ioc);

This API design has limitations: reading and writing must be done in the same
AioContext and moving between AioContexts involves a cumbersome sequence of API
calls that is not suitable for doing on a per-request basis.

There is no fundamental reason why a QIOChannel needs to run within the
same AioContext every time qio_channel_yield() is called. QIOChannel
only uses the AioContext while inside qio_channel_yield(). The rest of
the time, QIOChannel is independent of any AioContext.

In the new API, qio_channel_yield() queries the AioContext from the current
coroutine using qemu_coroutine_get_aio_context(). There is no need to
explicitly attach/detach AioContexts anymore and
qio_channel_attach_aio_context() and qio_channel_detach_aio_context() are gone.
One coroutine can read from the QIOChannel while another coroutine writes from
a different AioContext.

This API change allows the nbd block driver to use QIOChannel from any thread.
It's important to keep in mind that the block driver already synchronizes
QIOChannel access and ensures that two coroutines never read simultaneously or
write simultaneously.

This patch updates all users of qio_channel_attach_aio_context() to the
new API. Most conversions are simple, but vhost-user-server requires a
new qemu_coroutine_yield() call to quiesce the vu_client_trip()
coroutine when not attached to any AioContext.

While the API is has become simpler, there is one wart: QIOChannel has a
special case for the iohandler AioContext (used for handlers that must not run
in nested event loops). I didn't find an elegant way preserve that behavior, so
I added a new API called qio_channel_set_follow_coroutine_ctx(ioc, true|false)
for opting in to the new AioContext model. By default QIOChannel uses the
iohandler AioHandler. Code that formerly called
qio_channel_attach_aio_context() now calls
qio_channel_set_follow_coroutine_ctx(ioc, true) once after the QIOChannel is
created.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20230830224802.493686-5-stefanha@redhat.com>
[eblake: also fix migration/rdma.c]
Signed-off-by: Eric Blake <eblake@redhat.com>

16 files changed, 226 insertions, 126 deletions

diff --git a/block/nbd.c b/block/nbd.c
index 5322e66166..cc48580df7 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -352,7 +352,7 @@ int coroutine_fn nbd_co_do_establish_connection(BlockDriverState *bs,
     }
 
     qio_channel_set_blocking(s->ioc, false, NULL);
-    qio_channel_attach_aio_context(s->ioc, bdrv_get_aio_context(bs));
+    qio_channel_set_follow_coroutine_ctx(s->ioc, true);
 
     /* successfully connected */
     WITH_QEMU_LOCK_GUARD(&s->requests_lock) {
@@ -397,7 +397,6 @@ static void coroutine_fn GRAPH_RDLOCK nbd_reconnect_attempt(BDRVNBDState *s)
 
     /* Finalize previous connection if any */
     if (s->ioc) {
-        qio_channel_detach_aio_context(s->ioc);
         yank_unregister_function(BLOCKDEV_YANK_INSTANCE(s->bs->node_name),
                                  nbd_yank, s->bs);
         object_unref(OBJECT(s->ioc));
@@ -2089,10 +2088,6 @@ static void nbd_attach_aio_context(BlockDriverState *bs,
      * the reconnect_delay_timer cannot be active here.
      */
     assert(!s->reconnect_delay_timer);
-
-    if (s->ioc) {
-        qio_channel_attach_aio_context(s->ioc, new_context);
-    }
 }
 
 static void nbd_detach_aio_context(BlockDriverState *bs)
@@ -2101,10 +2096,6 @@ static void nbd_detach_aio_context(BlockDriverState *bs)
 
     assert(!s->open_timer);
     assert(!s->reconnect_delay_timer);
-
-    if (s->ioc) {
-        qio_channel_detach_aio_context(s->ioc);
-    }
 }
 
 static BlockDriver bdrv_nbd = {
diff --git a/include/io/channel-util.h b/include/io/channel-util.h
index a5d720d9a0..fa18a3756d 100644
--- a/include/io/channel-util.h
+++ b/include/io/channel-util.h
@@ -49,4 +49,27 @@
 QIOChannel *qio_channel_new_fd(int fd,
                                Error **errp);
 
+/**
+ * qio_channel_util_set_aio_fd_handler:
+ * @read_fd: the file descriptor for the read handler
+ * @read_ctx: the AioContext for the read handler
+ * @io_read: the read handler
+ * @write_fd: the file descriptor for the write handler
+ * @write_ctx: the AioContext for the write handler
+ * @io_write: the write handler
+ * @opaque: the opaque argument to the read and write handler
+ *
+ * Set the read and write handlers when @read_ctx and @write_ctx are non-NULL,
+ * respectively. To leave a handler in its current state, pass a NULL
+ * AioContext. To clear a handler, pass a non-NULL AioContext and a NULL
+ * handler.
+ */
+void qio_channel_util_set_aio_fd_handler(int read_fd,
+                                         AioContext *read_ctx,
+                                         IOHandler *io_read,
+                                         int write_fd,
+                                         AioContext *write_ctx,
+                                         IOHandler *io_write,
+                                         void *opaque);
+
 #endif /* QIO_CHANNEL_UTIL_H */
diff --git a/include/io/channel.h b/include/io/channel.h
index 229bf36910..5f9dbaab65 100644
--- a/include/io/channel.h
+++ b/include/io/channel.h
@@ -81,9 +81,11 @@ struct QIOChannel {
     Object parent;
     unsigned int features; /* bitmask of QIOChannelFeatures */
     char *name;
-    AioContext *ctx;
+    AioContext *read_ctx;
     Coroutine *read_coroutine;
+    AioContext *write_ctx;
     Coroutine *write_coroutine;
+    bool follow_coroutine_ctx;
 #ifdef _WIN32
     HANDLE event; /* For use with GSource on Win32 */
 #endif
@@ -140,8 +142,9 @@ struct QIOChannelClass {
                      int whence,
                      Error **errp);
     void (*io_set_aio_fd_handler)(QIOChannel *ioc,
-                                  AioContext *ctx,
+                                  AioContext *read_ctx,
                                   IOHandler *io_read,
+                                  AioContext *write_ctx,
                                   IOHandler *io_write,
                                   void *opaque);
     int (*io_flush)(QIOChannel *ioc,
@@ -499,6 +502,21 @@ int qio_channel_set_blocking(QIOChannel *ioc,
                              Error **errp);
 
 /**
+ * qio_channel_set_follow_coroutine_ctx:
+ * @ioc: the channel object
+ * @enabled: whether or not to follow the coroutine's AioContext
+ *
+ * If @enabled is true, calls to qio_channel_yield() use the current
+ * coroutine's AioContext. Usually this is desirable.
+ *
+ * If @enabled is false, calls to qio_channel_yield() use the global iohandler
+ * AioContext. This is may be used by coroutines that run in the main loop and
+ * do not wish to respond to I/O during nested event loops. This is the
+ * default for compatibility with code that is not aware of AioContexts.
+ */
+void qio_channel_set_follow_coroutine_ctx(QIOChannel *ioc, bool enabled);
+
+/**
  * qio_channel_close:
  * @ioc: the channel object
  * @errp: pointer to a NULL-initialized error object
@@ -704,41 +722,6 @@ GSource *qio_channel_add_watch_source(QIOChannel *ioc,
                                       GMainContext *context);
 
 /**
- * qio_channel_attach_aio_context:
- * @ioc: the channel object
- * @ctx: the #AioContext to set the handlers on
- *
- * Request that qio_channel_yield() sets I/O handlers on
- * the given #AioContext.  If @ctx is %NULL, qio_channel_yield()
- * uses QEMU's main thread event loop.
- *
- * You can move a #QIOChannel from one #AioContext to another even if
- * I/O handlers are set for a coroutine.  However, #QIOChannel provides
- * no synchronization between the calls to qio_channel_yield() and
- * qio_channel_attach_aio_context().
- *
- * Therefore you should first call qio_channel_detach_aio_context()
- * to ensure that the coroutine is not entered concurrently.  Then,
- * while the coroutine has yielded, call qio_channel_attach_aio_context(),
- * and then aio_co_schedule() to place the coroutine on the new
- * #AioContext.  The calls to qio_channel_detach_aio_context()
- * and qio_channel_attach_aio_context() should be protected with
- * aio_context_acquire() and aio_context_release().
- */
-void qio_channel_attach_aio_context(QIOChannel *ioc,
-                                    AioContext *ctx);
-
-/**
- * qio_channel_detach_aio_context:
- * @ioc: the channel object
- *
- * Disable any I/O handlers set by qio_channel_yield().  With the
- * help of aio_co_schedule(), this allows moving a coroutine that was
- * paused by qio_channel_yield() to another context.
- */
-void qio_channel_detach_aio_context(QIOChannel *ioc);
-
-/**
  * qio_channel_yield:
  * @ioc: the channel object
  * @condition: the I/O condition to wait for
@@ -785,8 +768,9 @@ void qio_channel_wait(QIOChannel *ioc,
 /**
  * qio_channel_set_aio_fd_handler:
  * @ioc: the channel object
- * @ctx: the AioContext to set the handlers on
+ * @read_ctx: the AioContext to set the read handler on or NULL
  * @io_read: the read handler
+ * @write_ctx: the AioContext to set the write handler on or NULL
  * @io_write: the write handler
  * @opaque: the opaque value passed to the handler
  *
@@ -794,10 +778,17 @@ void qio_channel_wait(QIOChannel *ioc,
  * be used by channel implementations to forward the handlers
  * to another channel (e.g. from #QIOChannelTLS to the
  * underlying socket).
+ *
+ * When @read_ctx is NULL, don't touch the read handler. When @write_ctx is
+ * NULL, don't touch the write handler. Note that setting the read handler
+ * clears the write handler, and vice versa, if they share the same AioContext.
+ * Therefore the caller must pass both handlers together when sharing the same
+ * AioContext.
  */
 void qio_channel_set_aio_fd_handler(QIOChannel *ioc,
-                                    AioContext *ctx,
+                                    AioContext *read_ctx,
                                     IOHandler *io_read,
+                                    AioContext *write_ctx,
                                     IOHandler *io_write,
                                     void *opaque);
 
diff --git a/include/qemu/vhost-user-server.h b/include/qemu/vhost-user-server.h
index b1c1cda886..64ad701015 100644
--- a/include/qemu/vhost-user-server.h
+++ b/include/qemu/vhost-user-server.h
@@ -43,6 +43,7 @@ typedef struct {
     unsigned int in_flight; /* atomic */
 
     /* Protected by ctx lock */
+    bool in_qio_channel_yield;
     bool wait_idle;
     VuDev vu_dev;
     QIOChannel *ioc; /* The I/O channel with the client */
diff --git a/io/channel-command.c b/io/channel-command.c
index 7ed726c802..6d5f64e146 100644
--- a/io/channel-command.c
+++ b/io/channel-command.c
@@ -20,6 +20,7 @@
 
 #include "qemu/osdep.h"
 #include "io/channel-command.h"
+#include "io/channel-util.h"
 #include "io/channel-watch.h"
 #include "qapi/error.h"
 #include "qemu/module.h"
@@ -331,14 +332,17 @@ static int qio_channel_command_close(QIOChannel *ioc,
 
 
 static void qio_channel_command_set_aio_fd_handler(QIOChannel *ioc,
-                                                   AioContext *ctx,
+                                                   AioContext *read_ctx,
                                                    IOHandler *io_read,
+                                                   AioContext *write_ctx,
                                                    IOHandler *io_write,
                                                    void *opaque)
 {
     QIOChannelCommand *cioc = QIO_CHANNEL_COMMAND(ioc);
-    aio_set_fd_handler(ctx, cioc->readfd, io_read, NULL, NULL, NULL, opaque);
-    aio_set_fd_handler(ctx, cioc->writefd, NULL, io_write, NULL, NULL, opaque);
+
+    qio_channel_util_set_aio_fd_handler(cioc->readfd, read_ctx, io_read,
+                                        cioc->writefd, write_ctx, io_write,
+                                        opaque);
 }
 
 
diff --git a/io/channel-file.c b/io/channel-file.c
index 8b5821f452..4a12c61886 100644
--- a/io/channel-file.c
+++ b/io/channel-file.c
@@ -20,6 +20,7 @@
 
 #include "qemu/osdep.h"
 #include "io/channel-file.h"
+#include "io/channel-util.h"
 #include "io/channel-watch.h"
 #include "qapi/error.h"
 #include "qemu/module.h"
@@ -192,13 +193,17 @@ static int qio_channel_file_close(QIOChannel *ioc,
 
 
 static void qio_channel_file_set_aio_fd_handler(QIOChannel *ioc,
-                                                AioContext *ctx,
+                                                AioContext *read_ctx,
                                                 IOHandler *io_read,
+                                                AioContext *write_ctx,
                                                 IOHandler *io_write,
                                                 void *opaque)
 {
     QIOChannelFile *fioc = QIO_CHANNEL_FILE(ioc);
-    aio_set_fd_handler(ctx, fioc->fd, io_read, io_write, NULL, NULL, opaque);
+
+    qio_channel_util_set_aio_fd_handler(fioc->fd, read_ctx, io_read,
+                                        fioc->fd, write_ctx, io_write,
+                                        opaque);
 }
 
 static GSource *qio_channel_file_create_watch(QIOChannel *ioc,
diff --git a/io/channel-null.c b/io/channel-null.c
index 4fafdb770d..ef99586348 100644
--- a/io/channel-null.c
+++ b/io/channel-null.c
@@ -128,8 +128,9 @@ qio_channel_null_close(QIOChannel *ioc,
 
 static void
 qio_channel_null_set_aio_fd_handler(QIOChannel *ioc G_GNUC_UNUSED,
-                                    AioContext *ctx G_GNUC_UNUSED,
+                                    AioContext *read_ctx G_GNUC_UNUSED,
                                     IOHandler *io_read G_GNUC_UNUSED,
+                                    AioContext *write_ctx G_GNUC_UNUSED,
                                     IOHandler *io_write G_GNUC_UNUSED,
                                     void *opaque G_GNUC_UNUSED)
 {
diff --git a/io/channel-socket.c b/io/channel-socket.c
index d99945ebec..02ffb51e99 100644
--- a/io/channel-socket.c
+++ b/io/channel-socket.c
@@ -22,6 +22,7 @@
 #include "qapi/qapi-visit-sockets.h"
 #include "qemu/module.h"
 #include "io/channel-socket.h"
+#include "io/channel-util.h"
 #include "io/channel-watch.h"
 #include "trace.h"
 #include "qapi/clone-visitor.h"
@@ -893,13 +894,17 @@ qio_channel_socket_shutdown(QIOChannel *ioc,
 }
 
 static void qio_channel_socket_set_aio_fd_handler(QIOChannel *ioc,
-                                                  AioContext *ctx,
+                                                  AioContext *read_ctx,
                                                   IOHandler *io_read,
+                                                  AioContext *write_ctx,
                                                   IOHandler *io_write,
                                                   void *opaque)
 {
     QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
-    aio_set_fd_handler(ctx, sioc->fd, io_read, io_write, NULL, NULL, opaque);
+
+    qio_channel_util_set_aio_fd_handler(sioc->fd, read_ctx, io_read,
+                                        sioc->fd, write_ctx, io_write,
+                                        opaque);
 }
 
 static GSource *qio_channel_socket_create_watch(QIOChannel *ioc,
diff --git a/io/channel-tls.c b/io/channel-tls.c
index 847d5297c3..58fe1aceee 100644
--- a/io/channel-tls.c
+++ b/io/channel-tls.c
@@ -388,14 +388,16 @@ static int qio_channel_tls_close(QIOChannel *ioc,
 }
 
 static void qio_channel_tls_set_aio_fd_handler(QIOChannel *ioc,
-                                               AioContext *ctx,
+                                               AioContext *read_ctx,
                                                IOHandler *io_read,
+                                               AioContext *write_ctx,
                                                IOHandler *io_write,
                                                void *opaque)
 {
     QIOChannelTLS *tioc = QIO_CHANNEL_TLS(ioc);
 
-    qio_channel_set_aio_fd_handler(tioc->master, ctx, io_read, io_write, opaque);
+    qio_channel_set_aio_fd_handler(tioc->master, read_ctx, io_read,
+            write_ctx, io_write, opaque);
 }
 
 typedef struct QIOChannelTLSSource QIOChannelTLSSource;
diff --git a/io/channel-util.c b/io/channel-util.c
index 848a7a43d6..4b340d46d7 100644
--- a/io/channel-util.c
+++ b/io/channel-util.c
@@ -36,3 +36,27 @@ QIOChannel *qio_channel_new_fd(int fd,
     }
     return ioc;
 }
+
+
+void qio_channel_util_set_aio_fd_handler(int read_fd,
+                                         AioContext *read_ctx,
+                                         IOHandler *io_read,
+                                         int write_fd,
+                                         AioContext *write_ctx,
+                                         IOHandler *io_write,
+                                         void *opaque)
+{
+    if (read_fd == write_fd && read_ctx == write_ctx) {
+        aio_set_fd_handler(read_ctx, read_fd, io_read, io_write,
+                NULL, NULL, opaque);
+    } else {
+        if (read_ctx) {
+            aio_set_fd_handler(read_ctx, read_fd, io_read, NULL,
+                    NULL, NULL, opaque);
+        }
+        if (write_ctx) {
+            aio_set_fd_handler(write_ctx, write_fd, NULL, io_write,
+                    NULL, NULL, opaque);
+        }
+    }
+}
diff --git a/io/channel.c b/io/channel.c
index c415f3fc88..86c5834510 100644
--- a/io/channel.c
+++ b/io/channel.c
@@ -365,6 +365,12 @@ int qio_channel_set_blocking(QIOChannel *ioc,
 }
 
 
+void qio_channel_set_follow_coroutine_ctx(QIOChannel *ioc, bool enabled)
+{
+    ioc->follow_coroutine_ctx = enabled;
+}
+
+
 int qio_channel_close(QIOChannel *ioc,
                       Error **errp)
 {
@@ -388,14 +394,16 @@ GSource *qio_channel_create_watch(QIOChannel *ioc,
 
 
 void qio_channel_set_aio_fd_handler(QIOChannel *ioc,
-                                    AioContext *ctx,
+                                    AioContext *read_ctx,
                                     IOHandler *io_read,
+                                    AioContext *write_ctx,
                                     IOHandler *io_write,
                                     void *opaque)
 {
     QIOChannelClass *klass = QIO_CHANNEL_GET_CLASS(ioc);
 
-    klass->io_set_aio_fd_handler(ioc, ctx, io_read, io_write, opaque);
+    klass->io_set_aio_fd_handler(ioc, read_ctx, io_read, write_ctx, io_write,
+            opaque);
 }
 
 guint qio_channel_add_watch_full(QIOChannel *ioc,
@@ -542,56 +550,101 @@ static void qio_channel_restart_write(void *opaque)
     aio_co_wake(co);
 }
 
-static void qio_channel_set_aio_fd_handlers(QIOChannel *ioc)
+static void coroutine_fn
+qio_channel_set_fd_handlers(QIOChannel *ioc, GIOCondition condition)
 {
-    IOHandler *rd_handler = NULL, *wr_handler = NULL;
-    AioContext *ctx;
+    AioContext *ctx = ioc->follow_coroutine_ctx ?
+        qemu_coroutine_get_aio_context(qemu_coroutine_self()) :
+        iohandler_get_aio_context();
+    AioContext *read_ctx = NULL;
+    IOHandler *io_read = NULL;
+    AioContext *write_ctx = NULL;
+    IOHandler *io_write = NULL;
 
-    if (ioc->read_coroutine) {
-        rd_handler = qio_channel_restart_read;
-    }
-    if (ioc->write_coroutine) {
-        wr_handler = qio_channel_restart_write;
+    if (condition == G_IO_IN) {
+        ioc->read_coroutine = qemu_coroutine_self();
+        ioc->read_ctx = ctx;
+        read_ctx = ctx;
+        io_read = qio_channel_restart_read;
+
+        /*
+         * Thread safety: if the other coroutine is set and its AioContext
+         * matches ours, then there is mutual exclusion between read and write
+         * because they share a single thread and it's safe to set both read
+         * and write fd handlers here. If the AioContext does not match ours,
+         * then both threads may run in parallel but there is no shared state
+         * to worry about.
+         */
+        if (ioc->write_coroutine && ioc->write_ctx == ctx) {
+            write_ctx = ctx;
+            io_write = qio_channel_restart_write;
+        }
+    } else if (condition == G_IO_OUT) {
+        ioc->write_coroutine = qemu_coroutine_self();
+        ioc->write_ctx = ctx;
+        write_ctx = ctx;
+        io_write = qio_channel_restart_write;
+        if (ioc->read_coroutine && ioc->read_ctx == ctx) {
+            read_ctx = ctx;
+            io_read = qio_channel_restart_read;
+        }
+    } else {
+        abort();
     }
 
-    ctx = ioc->ctx ? ioc->ctx : iohandler_get_aio_context();
-    qio_channel_set_aio_fd_handler(ioc, ctx, rd_handler, wr_handler, ioc);
+    qio_channel_set_aio_fd_handler(ioc, read_ctx, io_read,
+            write_ctx, io_write, ioc);
 }
 
-void qio_channel_attach_aio_context(QIOChannel *ioc,
-                                    AioContext *ctx)
+static void coroutine_fn
+qio_channel_clear_fd_handlers(QIOChannel *ioc, GIOCondition condition)
 {
-    assert(!ioc->read_coroutine);
-    assert(!ioc->write_coroutine);
-    ioc->ctx = ctx;
-}
+    AioContext *read_ctx = NULL;
+    IOHandler *io_read = NULL;
+    AioContext *write_ctx = NULL;
+    IOHandler *io_write = NULL;
+    AioContext *ctx;
 
-void qio_channel_detach_aio_context(QIOChannel *ioc)
-{
-    ioc->read_coroutine = NULL;
-    ioc->write_coroutine = NULL;
-    qio_channel_set_aio_fd_handlers(ioc);
-    ioc->ctx = NULL;
+    if (condition == G_IO_IN) {
+        ctx = ioc->read_ctx;
+        read_ctx = ctx;
+        io_read = NULL;
+        if (ioc->write_coroutine && ioc->write_ctx == ctx) {
+            write_ctx = ctx;
+            io_write = qio_channel_restart_write;
+        }
+    } else if (condition == G_IO_OUT) {
+        ctx = ioc->write_ctx;
+        write_ctx = ctx;
+        io_write = NULL;
+        if (ioc->read_coroutine && ioc->read_ctx == ctx) {
+            read_ctx = ctx;
+            io_read = qio_channel_restart_read;
+        }
+    } else {
+        abort();
+    }
+
+    qio_channel_set_aio_fd_handler(ioc, read_ctx, io_read,
+            write_ctx, io_write, ioc);
 }
 
 void coroutine_fn qio_channel_yield(QIOChannel *ioc,
                                     GIOCondition condition)
 {
-    AioContext *ioc_ctx = ioc->ctx ?: qemu_get_aio_context();
+    AioContext *ioc_ctx;
 
     assert(qemu_in_coroutine());
-    assert(in_aio_context_home_thread(ioc_ctx));
+    ioc_ctx = qemu_coroutine_get_aio_context(qemu_coroutine_self());
 
     if (condition == G_IO_IN) {
         assert(!ioc->read_coroutine);
-        ioc->read_coroutine = qemu_coroutine_self();
     } else if (condition == G_IO_OUT) {
         assert(!ioc->write_coroutine);
-        ioc->write_coroutine = qemu_coroutine_self();
     } else {
         abort();
     }
-    qio_channel_set_aio_fd_handlers(ioc);
+    qio_channel_set_fd_handlers(ioc, condition);
     qemu_coroutine_yield();
     assert(in_aio_context_home_thread(ioc_ctx));
 
@@ -599,11 +652,10 @@ void coroutine_fn qio_channel_yield(QIOChannel *ioc,
      * through the aio_fd_handlers. */
     if (condition == G_IO_IN) {
         assert(ioc->read_coroutine == NULL);
-        qio_channel_set_aio_fd_handlers(ioc);
     } else if (condition == G_IO_OUT) {
         assert(ioc->write_coroutine == NULL);
-        qio_channel_set_aio_fd_handlers(ioc);
     }
+    qio_channel_clear_fd_handlers(ioc, condition);
 }
 
 void qio_channel_wake_read(QIOChannel *ioc)
diff --git a/migration/channel-block.c b/migration/channel-block.c
index b7374363c3..fff8d87094 100644
--- a/migration/channel-block.c
+++ b/migration/channel-block.c
@@ -158,8 +158,9 @@ qio_channel_block_close(QIOChannel *ioc,
 
 static void
 qio_channel_block_set_aio_fd_handler(QIOChannel *ioc,
-                                     AioContext *ctx,
+                                     AioContext *read_ctx,
                                      IOHandler *io_read,
+                                     AioContext *write_ctx,
                                      IOHandler *io_write,
                                      void *opaque)
 {
diff --git a/migration/rdma.c b/migration/rdma.c
index ca430d319d..a2a3db35b1 100644
--- a/migration/rdma.c
+++ b/migration/rdma.c
@@ -3103,22 +3103,23 @@ static GSource *qio_channel_rdma_create_watch(QIOChannel *ioc,
 }
 
 static void qio_channel_rdma_set_aio_fd_handler(QIOChannel *ioc,
-                                                  AioContext *ctx,
-                                                  IOHandler *io_read,
-                                                  IOHandler *io_write,
-                                                  void *opaque)
+                                                AioContext *read_ctx,
+                                                IOHandler *io_read,
+                                                AioContext *write_ctx,
+                                                IOHandler *io_write,
+                                                void *opaque)
 {
     QIOChannelRDMA *rioc = QIO_CHANNEL_RDMA(ioc);
     if (io_read) {
-        aio_set_fd_handler(ctx, rioc->rdmain->recv_comp_channel->fd, io_read,
-                           io_write, NULL, NULL, opaque);
-        aio_set_fd_handler(ctx, rioc->rdmain->send_comp_channel->fd, io_read,
-                           io_write, NULL, NULL, opaque);
+        aio_set_fd_handler(read_ctx, rioc->rdmain->recv_comp_channel->fd,
+                           io_read, io_write, NULL, NULL, opaque);
+        aio_set_fd_handler(read_ctx, rioc->rdmain->send_comp_channel->fd,
+                           io_read, io_write, NULL, NULL, opaque);
     } else {
-        aio_set_fd_handler(ctx, rioc->rdmaout->recv_comp_channel->fd, io_read,
-                           io_write, NULL, NULL, opaque);
-        aio_set_fd_handler(ctx, rioc->rdmaout->send_comp_channel->fd, io_read,
-                           io_write, NULL, NULL, opaque);
+        aio_set_fd_handler(write_ctx, rioc->rdmaout->recv_comp_channel->fd,
+                           io_read, io_write, NULL, NULL, opaque);
+        aio_set_fd_handler(write_ctx, rioc->rdmaout->send_comp_channel->fd,
+                           io_read, io_write, NULL, NULL, opaque);
     }
 }
 
diff --git a/nbd/server.c b/nbd/server.c
index 8486b64b15..b5f93a20c9 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1333,6 +1333,7 @@ static coroutine_fn int nbd_negotiate(NBDClient *client, Error **errp)
      */
 
     qio_channel_set_blocking(client->ioc, false, NULL);
+    qio_channel_set_follow_coroutine_ctx(client->ioc, true);
 
     trace_nbd_negotiate_begin();
     memcpy(buf, "NBDMAGIC", 8);
@@ -1352,11 +1353,6 @@ static coroutine_fn int nbd_negotiate(NBDClient *client, Error **errp)
         return ret;
     }
 
-    /* Attach the channel to the same AioContext as the export */
-    if (client->exp && client->exp->common.ctx) {
-        qio_channel_attach_aio_context(client->ioc, client->exp->common.ctx);
-    }
-
     assert(!client->optlen);
     trace_nbd_negotiate_success();
 
@@ -1465,7 +1461,6 @@ void nbd_client_put(NBDClient *client)
          */
         assert(client->closing);
 
-        qio_channel_detach_aio_context(client->ioc);
         object_unref(OBJECT(client->sioc));
         object_unref(OBJECT(client->ioc));
         if (client->tlscreds) {
@@ -1544,8 +1539,6 @@ static void blk_aio_attached(AioContext *ctx, void *opaque)
     exp->common.ctx = ctx;
 
     QTAILQ_FOREACH(client, &exp->clients, next) {
-        qio_channel_attach_aio_context(client->ioc, ctx);
-
         assert(client->nb_requests == 0);
         assert(client->recv_coroutine == NULL);
         assert(client->send_coroutine == NULL);
@@ -1555,14 +1548,9 @@ static void blk_aio_attached(AioContext *ctx, void *opaque)
 static void blk_aio_detach(void *opaque)
 {
     NBDExport *exp = opaque;
-    NBDClient *client;
 
     trace_nbd_blk_aio_detach(exp->name, exp->common.ctx);
 
-    QTAILQ_FOREACH(client, &exp->clients, next) {
-        qio_channel_detach_aio_context(client->ioc);
-    }
-
     exp->common.ctx = NULL;
 }
 
diff --git a/scsi/qemu-pr-helper.c b/scsi/qemu-pr-helper.c
index ae44a816e1..c6c6347e9b 100644
--- a/scsi/qemu-pr-helper.c
+++ b/scsi/qemu-pr-helper.c
@@ -735,8 +735,7 @@ static void coroutine_fn prh_co_entry(void *opaque)
 
     qio_channel_set_blocking(QIO_CHANNEL(client->ioc),
                              false, NULL);
-    qio_channel_attach_aio_context(QIO_CHANNEL(client->ioc),
-                                   qemu_get_aio_context());
+    qio_channel_set_follow_coroutine_ctx(QIO_CHANNEL(client->ioc), true);
 
     /* A very simple negotiation for future extensibility.  No features
      * are defined so write 0.
@@ -796,7 +795,6 @@ static void coroutine_fn prh_co_entry(void *opaque)
     }
 
 out:
-    qio_channel_detach_aio_context(QIO_CHANNEL(client->ioc));
     object_unref(OBJECT(client->ioc));
     g_free(client);
 }
diff --git a/util/vhost-user-server.c b/util/vhost-user-server.c
index cd17fb5326..b4b6bf30a2 100644
--- a/util/vhost-user-server.c
+++ b/util/vhost-user-server.c
@@ -127,7 +127,14 @@ vu_message_read(VuDev *vu_dev, int conn_fd, VhostUserMsg *vmsg)
         if (rc < 0) {
             if (rc == QIO_CHANNEL_ERR_BLOCK) {
                 assert(local_err == NULL);
-                qio_channel_yield(ioc, G_IO_IN);
+                if (server->ctx) {
+                    server->in_qio_channel_yield = true;
+                    qio_channel_yield(ioc, G_IO_IN);
+                    server->in_qio_channel_yield = false;
+                } else {
+                    /* Wait until attached to an AioContext again */
+                    qemu_coroutine_yield();
+                }
                 continue;
             } else {
                 error_report_err(local_err);
@@ -278,7 +285,7 @@ set_watch(VuDev *vu_dev, int fd, int vu_evt,
         vu_fd_watch->fd = fd;
         vu_fd_watch->cb = cb;
         qemu_socket_set_nonblock(fd);
-        aio_set_fd_handler(server->ioc->ctx, fd, kick_handler,
+        aio_set_fd_handler(server->ctx, fd, kick_handler,
                            NULL, NULL, NULL, vu_fd_watch);
         vu_fd_watch->vu_dev = vu_dev;
         vu_fd_watch->pvt = pvt;
@@ -299,7 +306,7 @@ static void remove_watch(VuDev *vu_dev, int fd)
     if (!vu_fd_watch) {
         return;
     }
-    aio_set_fd_handler(server->ioc->ctx, fd, NULL, NULL, NULL, NULL, NULL);
+    aio_set_fd_handler(server->ctx, fd, NULL, NULL, NULL, NULL, NULL);
 
     QTAILQ_REMOVE(&server->vu_fd_watches, vu_fd_watch, next);
     g_free(vu_fd_watch);
@@ -344,6 +351,8 @@ static void vu_accept(QIONetListener *listener, QIOChannelSocket *sioc,
     /* TODO vu_message_write() spins if non-blocking! */
     qio_channel_set_blocking(server->ioc, false, NULL);
 
+    qio_channel_set_follow_coroutine_ctx(server->ioc, true);
+
     server->co_trip = qemu_coroutine_create(vu_client_trip, server);
 
     aio_context_acquire(server->ctx);
@@ -399,13 +408,12 @@ void vhost_user_server_attach_aio_context(VuServer *server, AioContext *ctx)
         return;
     }
 
-    qio_channel_attach_aio_context(server->ioc, ctx);
-
     QTAILQ_FOREACH(vu_fd_watch, &server->vu_fd_watches, next) {
         aio_set_fd_handler(ctx, vu_fd_watch->fd, kick_handler, NULL,
                            NULL, NULL, vu_fd_watch);
     }
 
+    assert(!server->in_qio_channel_yield);
     aio_co_schedule(ctx, server->co_trip);
 }
 
@@ -419,11 +427,16 @@ void vhost_user_server_detach_aio_context(VuServer *server)
             aio_set_fd_handler(server->ctx, vu_fd_watch->fd,
                                NULL, NULL, NULL, NULL, vu_fd_watch);
         }
-
-        qio_channel_detach_aio_context(server->ioc);
     }
 
     server->ctx = NULL;
+
+    if (server->ioc) {
+        if (server->in_qio_channel_yield) {
+            /* Stop receiving the next vhost-user message */
+            qio_channel_wake_read(server->ioc);
+        }
+    }
 }
 
 bool vhost_user_server_start(VuServer *server,