feat(vhost-user): add a device-type agnostic vhost-user frontend

CHANGELOG.md

@@ -82,6 +82,11 @@ and this project adheres to
   Terminating a connection now also discards its TX buffer, so the device stops
   advertising `EPOLLOUT` for a host stream it will never write to again, which
   could otherwise busy-spin the event thread indefinitely.
+- [#6083](https://github.com/firecracker-microvm/firecracker/pull/6083): Fixed a
+  vhost-user-block device backed by a readonly backend not being treated as
+  readonly. The `VIRTIO_BLK_F_RO` check read the acked feature set after it had
+  been narrowed to the vhost-user protocol bit, so it never matched, and a
+  readonly vhost-user root device was given `rw` on the guest kernel cmdline.
 - [#6086](https://github.com/firecracker-microvm/firecracker/pull/6086),
   [#6143](https://github.com/firecracker-microvm/firecracker/pull/6143): Fixed a
   deadlock in the logger: a signal handler that logs while the interrupted

src/vmm/src/devices/virtio/block/vhost_user/device.rs

@@ -205,11 +205,13 @@ impl<T: VhostUserHandleBackend> VhostUserBlockImpl<T> {
             u64_to_usize(NUM_QUEUES)];
         let device_state = DeviceState::Inactive;
 
+        // Read before `acked_features` is narrowed to the protocol bit below.
+        let read_only = acked_features & (1 << VIRTIO_BLK_F_RO) != 0;
+
         // We negotiated features with backend. Now these acked_features
         // are available for guest driver to choose from.
         let avail_features = acked_features;
         let acked_features = acked_features & VhostUserVirtioFeatures::PROTOCOL_FEATURES.bits();
-        let read_only = acked_features & (1 << VIRTIO_BLK_F_RO) != 0;
         let vhost_user_block_metrics_name = format!("block_{}", config.drive_id);
 
         let metrics = VhostUserMetricsPerDevice::alloc(vhost_user_block_metrics_name);
@@ -657,7 +659,7 @@ mod tests {
             VhostUserHeaderFlag::empty().bits()
         );
         assert!(!vhost_block.root_device);
-        assert!(!vhost_block.read_only);
+        assert!(vhost_block.read_only);
         assert_eq!(vhost_block.config_space, vec![0x69, 0x69, 0x69]);
 
         // Test some `VirtioDevice` methods