[PATCH 1/4] vhost: Re-enable vrings after setting features

[Hanna Czenczek]

If the back-end supports the VHOST_USER_F_PROTOCOL_FEATURES feature,
setting the vhost features will set this feature, too.  Doing so
disables all vrings, which may not be intended.

For example, enabling or disabling logging during migration requires
setting those features (to set or unset VHOST_F_LOG_ALL), which will
automatically disable all vrings.  In either case, the VM is running
(disabling logging is done after a failed or cancelled migration, and
only once the VM is running again, see comment in
memory_global_dirty_log_stop()), so the vrings should really be enabled.
As a result, the back-end seems to hang.

To fix this, we must remember whether the vrings are supposed to be
enabled, and, if so, re-enable them after a SET_FEATURES call that set
VHOST_USER_F_PROTOCOL_FEATURES.

It seems less than ideal that there is a short period in which the VM is
running but the vrings will be stopped (between SET_FEATURES and
SET_VRING_ENABLE).  To fix this, we would need to change the protocol,
e.g. by introducing a new flag or vhost-user protocol feature to disable
disabling vrings whenever VHOST_USER_F_PROTOCOL_FEATURES is set, or add
new functions for setting/clearing singular feature bits (so that
F_LOG_ALL can be set/cleared without touching F_PROTOCOL_FEATURES).

Even with such a potential addition to the protocol, we still need this
fix here, because we cannot expect that back-ends will implement this
addition.

Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
 include/hw/virtio/vhost.h | 10 ++++++++++
 hw/virtio/vhost.c         | 13 +++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index a52f273347..2fe02ed5d4 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -90,6 +90,16 @@ struct vhost_dev {
     int vq_index_end;
     /* if non-zero, minimum required value for max_queues */
     int num_queues;
+
+    /*
+     * Whether the virtqueues are supposed to be enabled (via
+     * SET_VRING_ENABLE).  Setting the features (e.g. for
+     * enabling/disabling logging) will disable all virtqueues if
+     * VHOST_USER_F_PROTOCOL_FEATURES is set, so then we need to
+     * re-enable them if this field is set.
+     */
+    bool enable_vqs;
+
     /**
      * vhost feature handling requires matching the feature set
      * offered by a backend which may be a subset of the total
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index a266396576..cbff589efa 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -50,6 +50,8 @@ static unsigned int used_memslots;
 static QLIST_HEAD(, vhost_dev) vhost_devices =
     QLIST_HEAD_INITIALIZER(vhost_devices);
 
+static int vhost_dev_set_vring_enable(struct vhost_dev *hdev, int enable);
+
 bool vhost_has_free_slot(void)
 {
     unsigned int slots_limit = ~0U;
@@ -899,6 +901,15 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
         }
     }
 
+    if (dev->enable_vqs) {
+        /*
+         * Setting VHOST_USER_F_PROTOCOL_FEATURES would have disabled all
+         * virtqueues, even if that was not intended; re-enable them if
+         * necessary.
+         */
+        vhost_dev_set_vring_enable(dev, true);
+    }
+
 out:
     return r;
 }
@@ -1896,6 +1907,8 @@ int vhost_dev_get_inflight(struct vhost_dev *dev, 
uint16_t queue_size,
 
 static int vhost_dev_set_vring_enable(struct vhost_dev *hdev, int enable)
 {
+    hdev->enable_vqs = enable;
+
     if (!hdev->vhost_ops->vhost_set_vring_enable) {
         return 0;
     }
-- 
2.39.1