devices: vhost-user: forbid packed virtqueues temporarily

VHOST_USER_SET_VRING_BASE and VHOST_USER_GET_VRING_BASE have different
request/response formats if packed queues have been negotiated and we
don't support that yet. Forbid it for now in the vhost-user frontend and
backend to keep people from tripping on it.

Bug: 331466964
Change-Id: I7a064ab0d4677cec465baa444c308c84d87e00b2
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/5397650
Commit-Queue: Frederick Mayle <fmayle@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>

devices/src/virtio/vhost_user_frontend/mod.rs

@@ -122,10 +122,21 @@ impl VhostUserFrontend {
         connection: vmm_vhost::SystemStream,
         device_type: DeviceType,
         max_queue_size: Option<u16>,
-        base_features: u64,
+        mut base_features: u64,
         cfg: Option<&[u8]>,
         pci_address: Option<PciAddress>,
     ) -> Result<VhostUserFrontend> {
+        // Don't allow packed queues even if requested. We don't handle them properly yet at the
+        // protocol layer.
+        // TODO: b/331466964 - Remove once packed queue support is added to BackendClient.
+        if base_features & (1 << virtio_sys::virtio_config::VIRTIO_F_RING_PACKED) != 0 {
+            base_features &= !(1 << virtio_sys::virtio_config::VIRTIO_F_RING_PACKED);
+            base::warn!(
+                "VIRTIO_F_RING_PACKED requested, but not yet supported by vhost-user frontend. \
+                Automatically disabled."
+            );
+        }
+
         #[cfg(windows)]
         let backend_pid = connection.target_pid();
 

third_party/vmm_vhost/src/backend_client.rs

@@ -188,6 +188,7 @@ impl BackendClient {
     }
 
     /// Set the first index to look for available descriptors.
+    // TODO: b/331466964 - Arguments and message format are wrong for packed queues.
     pub fn set_vring_base(&self, queue_index: usize, base: u16) -> Result<()> {
         let val = VhostUserVringState::new(queue_index as u32, base.into());
         let hdr = self.send_request_with_body(FrontendReq::SET_VRING_BASE, &val, None)?;
@@ -195,6 +196,7 @@ impl BackendClient {
     }
 
     /// Get the available vring base offset.
+    // TODO: b/331466964 - Return type is wrong for packed queues.
     pub fn get_vring_base(&self, queue_index: usize) -> Result<u32> {
         let req = VhostUserVringState::new(queue_index as u32, 0);
         let hdr = self.send_request_with_body(FrontendReq::GET_VRING_BASE, &req, None)?;

third_party/vmm_vhost/src/backend_server.rs

@@ -41,7 +41,9 @@ pub trait Backend {
         available: u64,
         log: u64,
     ) -> Result<()>;
+    // TODO: b/331466964 - Argument type is wrong for packed queues.
     fn set_vring_base(&mut self, index: u32, base: u32) -> Result<()>;
+    // TODO: b/331466964 - Return type is wrong for packed queues.
     fn get_vring_base(&mut self, index: u32) -> Result<VhostUserVringState>;
     fn set_vring_kick(&mut self, index: u8, fd: Option<File>) -> Result<()>;
     fn set_vring_call(&mut self, index: u8, fd: Option<File>) -> Result<()>;
@@ -377,14 +379,26 @@ impl<S: Backend> BackendServer<S> {
             }
             Ok(FrontendReq::GET_FEATURES) => {
                 self.check_request_size(&hdr, size, 0)?;
-                let features = self.backend.get_features()?;
+                let mut features = self.backend.get_features()?;
+
+                // Don't advertise packed queues even if the device does. We don't handle them
+                // properly yet at the protocol layer.
+                // TODO: b/331466964 - Remove once support is added.
+                features &= !(1 << VIRTIO_F_RING_PACKED);
+
                 let msg = VhostUserU64::new(features);
                 self.send_reply_message(&hdr, &msg)?;
                 self.virtio_features = features;
                 self.update_reply_ack_flag();
             }
             Ok(FrontendReq::SET_FEATURES) => {
-                let msg = self.extract_request_body::<VhostUserU64>(&hdr, size, &buf)?;
+                let mut msg = self.extract_request_body::<VhostUserU64>(&hdr, size, &buf)?;
+
+                // Don't allow packed queues even if the device does. We don't handle them
+                // properly yet at the protocol layer.
+                // TODO: b/331466964 - Remove once support is added.
+                msg.value &= !(1 << VIRTIO_F_RING_PACKED);
+
                 let res = self.backend.set_features(msg.value);
                 self.acked_virtio_features = msg.value;
                 self.update_reply_ack_flag();

third_party/vmm_vhost/src/message.rs

@@ -394,6 +394,8 @@ impl<T: Req> VhostUserMsgValidator for VhostUserMsgHeader<T> {
     }
 }
 
+pub const VIRTIO_F_RING_PACKED: u32 = 34;
+
 /// Virtio feature flag for the vhost-user protocol features.
 pub const VHOST_USER_F_PROTOCOL_FEATURES: u32 = 30;