virtio-devices: iommu: Drop unrecognised requests without writing reply

The virtio spec requires the device to leave the reply buffer
untouched and report a used length of zero for an unrecognised
request type, so the driver can tell the request was not handled.

Signed-off-by: Rob Bradford <rbradford@meta.com>
Assisted-by: Claude:claude-opus-4-7

virtio-devices/src/iommu.rs

@@ -388,6 +388,7 @@ impl Request {
         let mut reply: Vec<u8> = Vec::new();
         let mut status = VIRTIO_IOMMU_S_OK;
         let mut hdr_len = 0;
+        let mut unrecognised_type = false;
 
         let result = (|| {
             match req_head.type_ {
@@ -679,13 +680,19 @@ impl Request {
                     hdr_len = PROBE_PROP_SIZE;
                 }
                 _ => {
-                    status = VIRTIO_IOMMU_S_INVAL;
+                    unrecognised_type = true;
                     return Err(Error::InvalidRequest);
                 }
             }
             Ok(())
         })();
 
+        // virtio spec: unrecognised request types must not have the reply
+        // buffer written and must report a used length of zero.
+        if unrecognised_type {
+            return Ok(0);
+        }
+
         let status_desc = desc_chain.next().ok_or(Error::DescriptorChainTooShort)?;
 
         // The status MUST always be writable