virtio-devices: fix barrier handling in virtio-blk

When configuring multiple queues for a virtio device, the guest can
activate between 1 and the configured amount of queues. The firmware,
for example, may activate only one queue, while a Linux guest would
likely activate all available queues.

The constructor of virtio-blk initializes the `paused_sync` barrier
using the configured queue count (plus one for the main thread). This
can be wrong if the guest enable a different number of queues at
activation time, which can make pause hang. Thus, we now recompute the
barrier size from the queues that are actually activated.

On-behalf-of: SAP sebastian.eydam@sap.com
Signed-off-by: Sebastian Eydam <sebastian.eydam@cyberus-technology.de>

virtio-devices/src/block.rs

@@ -1087,6 +1087,9 @@ impl VirtioDevice for Block {
         }
         self.common.activate(&queues, interrupt_cb.clone())?;
 
+        // Recompute the barrier size from the queues that are actually activated.
+        self.common.paused_sync = Some(Arc::new(Barrier::new(queues.len() + 1)));
+
         self.update_writeback();
 
         let mut epoll_threads = Vec::new();