Introduce Mutex<T: DeviceIoMut> adapter for DeviceIo

Many device backend drivers will mutate itself when handling IO
requests. The DeviceIo trait assumes interior mutability, but it's
a little complex to support interior mutability. So introduce the
Mutex<T: DeviceIoMut> adapter to ease device backend driver
implementations. And the Mutex<T: DeviceIoMut> adapter is an zero
overhead abstraction without performance penalty.

Liu Jiang <[email protected]>

Author: jiangliu

coverage_config.json

@@ -1,5 +1,5 @@
 {
-  "coverage_score": 79.9,
+  "coverage_score": 79.5,
   "exclude_path": "",
   "crate_features": ""
 }

src/lib.rs

@@ -4,6 +4,7 @@
 //! rust-vmm device model.
 
 use std::cmp::{Ord, Ordering, PartialOrd};
+use std::sync::Mutex;
 
 pub mod device_manager;
 pub mod resources;
@@ -68,13 +69,16 @@ impl PartialOrd for IoAddress {
     }
 }
 
-/// Device IO trait.
+/// Device IO trait adopting interior mutability pattern.
+///
 /// A device supporting memory based I/O should implement this trait, then
 /// register itself against the different IO type ranges it handles.
 /// The VMM will then dispatch IO (PIO or MMIO) VM exits by calling into the
 /// registered devices read or write method from this trait.
-/// The DeviceIo trait adopts the interior mutability pattern
-/// so we can get a real multiple threads handling.
+///
+/// The DeviceIo trait adopts the interior mutability pattern so we can get a real concurrent
+/// multiple threads handling. For device backend drivers not focusing on high performance,
+/// they may use the Mutex<T: DeviceIoMut> adapter to simplify implementation.
 pub trait DeviceIo: Send {
     /// Read from the guest physical address `base`, starting at `offset`.
     /// Result is placed in `data`.
@@ -83,3 +87,66 @@ pub trait DeviceIo: Send {
     /// Write `data` to the guest physical address `base`, starting from `offset`.
     fn write(&self, base: IoAddress, offset: IoAddress, data: &[u8]);
 }
+
+/// Device IO trait without interior mutability.
+///
+/// Many device backend drivers will mutate itself when handling IO requests. The DeviceIo trait
+/// assumes interior mutability, but it's a little complex to support interior mutability. So
+/// the Mutex<T: DeviceIoMut> adapter may be used to ease device backend driver implementations.
+/// The Mutex<T: DeviceIoMut> adapter is an zero overhead abstraction without performance penalty.
+pub trait DeviceIoMut: Send {
+    /// Read from the guest physical address `base`, starting at `offset`.
+    /// Result is placed in `data`.
+    fn read(&mut self, base: IoAddress, offset: IoAddress, data: &mut [u8]);
+
+    /// Write `data` to the guest physical address `base`, starting from `offset`.
+    fn write(&mut self, base: IoAddress, offset: IoAddress, data: &[u8]);
+}
+
+impl<T: DeviceIoMut> DeviceIo for Mutex<T> {
+    fn read(&self, base: IoAddress, offset: IoAddress, data: &mut [u8]) {
+        // Safe to unwrap() because we don't expect poisoned lock here.
+        self.lock().unwrap().read(base, offset, data)
+    }
+
+    fn write(&self, base: IoAddress, offset: IoAddress, data: &[u8]) {
+        // Safe to unwrap() because we don't expect poisoned lock here.
+        self.lock().unwrap().write(base, offset, data)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::sync::Arc;
+
+    #[derive(Default)]
+    struct MockDevice {
+        data: u8,
+    }
+
+    impl DeviceIoMut for MockDevice {
+        fn read(&mut self, _base: IoAddress, _offset: IoAddress, data: &mut [u8]) {
+            data[0] = self.data;
+        }
+
+        fn write(&mut self, _base: IoAddress, _offset: IoAddress, data: &[u8]) {
+            self.data = data[0];
+        }
+    }
+
+    fn register_device(device: Arc<dyn DeviceIo>) {
+        device.write(IoAddress::Mmio(0), IoAddress::Mmio(0), &[0x10u8]);
+        let mut buf = [0x0u8];
+        device.read(IoAddress::Mmio(0), IoAddress::Mmio(0), &mut buf);
+        assert_eq!(buf[0], 0x10);
+    }
+
+    #[test]
+    fn test_device_io_mut_adapter() {
+        let device_mut = Arc::new(Mutex::new(MockDevice::default()));
+
+        register_device(device_mut.clone());
+        assert_eq!(device_mut.lock().unwrap().data, 0x010);
+    }
+}