Introduce VhostBackendMut trait

Originally the VhostBackend trait is designed to take a mutable self,
which causes a common usage pattern Arc<RwLock<T: VhostBackend>>.
This pattern may enforce serialization among multiple threads. So
rename the original VhostBackend as VhostBackendMut, and introduce a
new VhostBackend trait with interior mutability to improve performance
by removing the serialization.

Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>

4 files changed, 354 insertions, 36 deletions

diff --git a/src/backend.rs b/src/backend.rs
index 2d1a4a2..9dafef7 100644
--- a/src/backend.rs
+++ b/src/backend.rs
@@ -1,4 +1,4 @@
-// Copyright (C) 2019 Alibaba Cloud Computing. All rights reserved.
+// Copyright (C) 2019-2021 Alibaba Cloud. All rights reserved.
 // SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
 //
 // Portions Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
@@ -9,14 +9,18 @@
 
 //! Common traits and structs for vhost-kern and vhost-user backend drivers.
 
-use super::Result;
+use std::cell::RefCell;
 use std::os::unix::io::RawFd;
+use std::sync::RwLock;
+
 use vmm_sys_util::eventfd::EventFd;
 
+use super::Result;
+
 /// Maximum number of memory regions supported.
 pub const VHOST_MAX_MEMORY_REGIONS: usize = 255;
 
-/// Vring/virtque configuration data.
+/// Vring configuration data.
 pub struct VringConfigData {
     /// Maximum queue size supported by the driver.
     pub queue_max_size: u16,
@@ -65,22 +69,109 @@ pub struct VhostUserMemoryRegionInfo {
     pub userspace_addr: u64,
     /// Optional offset where region starts in the mapped memory.
     pub mmap_offset: u64,
-    /// Optional file diescriptor for mmap
+    /// Optional file descriptor for mmap.
     pub mmap_handle: RawFd,
 }
 
-/// An interface for setting up vhost-based backend drivers.
+/// An interface for setting up vhost-based backend drivers with interior mutability.
 ///
 /// Vhost devices are subset of virtio devices, which improve virtio device's performance by
 /// delegating data plane operations to dedicated IO service processes. Vhost devices use the
 /// same virtqueue layout as virtio devices to allow vhost devices to be mapped directly to
 /// virtio devices.
+///
 /// The purpose of vhost is to implement a subset of a virtio device's functionality outside the
 /// VMM process. Typically fast paths for IO operations are delegated to the dedicated IO service
 /// processes, and slow path for device configuration are still handled by the VMM process. It may
 /// also be used to control access permissions of virtio backend devices.
 pub trait VhostBackend: std::marker::Sized {
     /// Get a bitmask of supported virtio/vhost features.
+    fn get_features(&self) -> Result<u64>;
+
+    /// Inform the vhost subsystem which features to enable.
+    /// This should be a subset of supported features from get_features().
+    ///
+    /// # Arguments
+    /// * `features` - Bitmask of features to set.
+    fn set_features(&self, features: u64) -> Result<()>;
+
+    /// Set the current process as the owner of the vhost backend.
+    /// This must be run before any other vhost commands.
+    fn set_owner(&self) -> Result<()>;
+
+    /// Used to be sent to request disabling all rings
+    /// This is no longer used.
+    fn reset_owner(&self) -> Result<()>;
+
+    /// Set the guest memory mappings for vhost to use.
+    fn set_mem_table(&self, regions: &[VhostUserMemoryRegionInfo]) -> Result<()>;
+
+    /// Set base address for page modification logging.
+    fn set_log_base(&self, base: u64, fd: Option<RawFd>) -> Result<()>;
+
+    /// Specify an eventfd file descriptor to signal on log write.
+    fn set_log_fd(&self, fd: RawFd) -> Result<()>;
+
+    /// Set the number of descriptors in the vring.
+    ///
+    /// # Arguments
+    /// * `queue_index` - Index of the queue to set descriptor count for.
+    /// * `num` - Number of descriptors in the queue.
+    fn set_vring_num(&self, queue_index: usize, num: u16) -> Result<()>;
+
+    /// Set the addresses for a given vring.
+    ///
+    /// # Arguments
+    /// * `queue_index` - Index of the queue to set addresses for.
+    /// * `config_data` - Configuration data for a vring.
+    fn set_vring_addr(&self, queue_index: usize, config_data: &VringConfigData) -> Result<()>;
+
+    /// Set the first index to look for available descriptors.
+    ///
+    /// # Arguments
+    /// * `queue_index` - Index of the queue to modify.
+    /// * `num` - Index where available descriptors start.
+    fn set_vring_base(&self, queue_index: usize, base: u16) -> Result<()>;
+
+    /// Get the available vring base offset.
+    fn get_vring_base(&self, queue_index: usize) -> Result<u32>;
+
+    /// Set the eventfd to trigger when buffers have been used by the host.
+    ///
+    /// # Arguments
+    /// * `queue_index` - Index of the queue to modify.
+    /// * `fd` - EventFd to trigger.
+    fn set_vring_call(&self, queue_index: usize, fd: &EventFd) -> Result<()>;
+
+    /// Set the eventfd that will be signaled by the guest when buffers are
+    /// available for the host to process.
+    ///
+    /// # Arguments
+    /// * `queue_index` - Index of the queue to modify.
+    /// * `fd` - EventFd that will be signaled from guest.
+    fn set_vring_kick(&self, queue_index: usize, fd: &EventFd) -> Result<()>;
+
+    /// Set the eventfd that will be signaled by the guest when error happens.
+    ///
+    /// # Arguments
+    /// * `queue_index` - Index of the queue to modify.
+    /// * `fd` - EventFd that will be signaled from guest.
+    fn set_vring_err(&self, queue_index: usize, fd: &EventFd) -> Result<()>;
+}
+
+/// An interface for setting up vhost-based backend drivers.
+///
+/// Vhost devices are subset of virtio devices, which improve virtio device's performance by
+/// delegating data plane operations to dedicated IO service processes. Vhost devices use the
+/// same virtqueue layout as virtio devices to allow vhost devices to be mapped directly to
+/// virtio devices.
+///
+/// The purpose of vhost is to implement a subset of a virtio device's functionality outside the
+/// VMM process. Typically fast paths for IO operations are delegated to the dedicated IO service
+/// processes, and slow path for device configuration are still handled by the VMM process. It may
+/// also be used to control access permissions of virtio backend devices.
+pub trait VhostBackendMut: std::marker::Sized {
+    /// Get a bitmask of supported virtio/vhost features.
     fn get_features(&mut self) -> Result<u64>;
 
     /// Inform the vhost subsystem which features to enable.
@@ -154,10 +245,237 @@ pub trait VhostBackend: std::marker::Sized {
     fn set_vring_err(&mut self, queue_index: usize, fd: &EventFd) -> Result<()>;
 }
 
+impl<T: VhostBackendMut> VhostBackend for RwLock<T> {
+    fn get_features(&self) -> Result<u64> {
+        self.write().unwrap().get_features()
+    }
+
+    fn set_features(&self, features: u64) -> Result<()> {
+        self.write().unwrap().set_features(features)
+    }
+
+    fn set_owner(&self) -> Result<()> {
+        self.write().unwrap().set_owner()
+    }
+
+    fn reset_owner(&self) -> Result<()> {
+        self.write().unwrap().reset_owner()
+    }
+
+    fn set_mem_table(&self, regions: &[VhostUserMemoryRegionInfo]) -> Result<()> {
+        self.write().unwrap().set_mem_table(regions)
+    }
+
+    fn set_log_base(&self, base: u64, fd: Option<RawFd>) -> Result<()> {
+        self.write().unwrap().set_log_base(base, fd)
+    }
+
+    fn set_log_fd(&self, fd: RawFd) -> Result<()> {
+        self.write().unwrap().set_log_fd(fd)
+    }
+
+    fn set_vring_num(&self, queue_index: usize, num: u16) -> Result<()> {
+        self.write().unwrap().set_vring_num(queue_index, num)
+    }
+
+    fn set_vring_addr(&self, queue_index: usize, config_data: &VringConfigData) -> Result<()> {
+        self.write()
+            .unwrap()
+            .set_vring_addr(queue_index, config_data)
+    }
+
+    fn set_vring_base(&self, queue_index: usize, base: u16) -> Result<()> {
+        self.write().unwrap().set_vring_base(queue_index, base)
+    }
+
+    fn get_vring_base(&self, queue_index: usize) -> Result<u32> {
+        self.write().unwrap().get_vring_base(queue_index)
+    }
+
+    fn set_vring_call(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
+        self.write().unwrap().set_vring_call(queue_index, fd)
+    }
+
+    fn set_vring_kick(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
+        self.write().unwrap().set_vring_kick(queue_index, fd)
+    }
+
+    fn set_vring_err(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
+        self.write().unwrap().set_vring_err(queue_index, fd)
+    }
+}
+
+impl<T: VhostBackendMut> VhostBackend for RefCell<T> {
+    fn get_features(&self) -> Result<u64> {
+        self.borrow_mut().get_features()
+    }
+
+    fn set_features(&self, features: u64) -> Result<()> {
+        self.borrow_mut().set_features(features)
+    }
+
+    fn set_owner(&self) -> Result<()> {
+        self.borrow_mut().set_owner()
+    }
+
+    fn reset_owner(&self) -> Result<()> {
+        self.borrow_mut().reset_owner()
+    }
+
+    fn set_mem_table(&self, regions: &[VhostUserMemoryRegionInfo]) -> Result<()> {
+        self.borrow_mut().set_mem_table(regions)
+    }
+
+    fn set_log_base(&self, base: u64, fd: Option<RawFd>) -> Result<()> {
+        self.borrow_mut().set_log_base(base, fd)
+    }
+
+    fn set_log_fd(&self, fd: RawFd) -> Result<()> {
+        self.borrow_mut().set_log_fd(fd)
+    }
+
+    fn set_vring_num(&self, queue_index: usize, num: u16) -> Result<()> {
+        self.borrow_mut().set_vring_num(queue_index, num)
+    }
+
+    fn set_vring_addr(&self, queue_index: usize, config_data: &VringConfigData) -> Result<()> {
+        self.borrow_mut().set_vring_addr(queue_index, config_data)
+    }
+
+    fn set_vring_base(&self, queue_index: usize, base: u16) -> Result<()> {
+        self.borrow_mut().set_vring_base(queue_index, base)
+    }
+
+    fn get_vring_base(&self, queue_index: usize) -> Result<u32> {
+        self.borrow_mut().get_vring_base(queue_index)
+    }
+
+    fn set_vring_call(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
+        self.borrow_mut().set_vring_call(queue_index, fd)
+    }
+
+    fn set_vring_kick(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
+        self.borrow_mut().set_vring_kick(queue_index, fd)
+    }
+
+    fn set_vring_err(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
+        self.borrow_mut().set_vring_err(queue_index, fd)
+    }
+}
 #[cfg(test)]
 mod tests {
     use VringConfigData;
 
+    struct MockBackend {}
+
+    impl VhostBackendMut for MockBackend {
+        fn get_features(&mut self) -> Result<u64> {
+            Ok(0x1)
+        }
+
+        fn set_features(&mut self, features: u64) -> Result<()> {
+            assert_eq!(features, 0x1);
+            Ok(())
+        }
+
+        fn set_owner(&mut self) -> Result<()> {
+            Ok(())
+        }
+
+        fn reset_owner(&mut self) -> Result<()> {
+            Ok(())
+        }
+
+        fn set_mem_table(&mut self, _regions: &[VhostUserMemoryRegionInfo]) -> Result<()> {
+            Ok(())
+        }
+
+        fn set_log_base(&mut self, base: u64, fd: Option<RawFd>) -> Result<()> {
+            assert_eq!(base, 0x100);
+            assert_eq!(fd, Some(100));
+            Ok(())
+        }
+
+        fn set_log_fd(&mut self, fd: RawFd) -> Result<()> {
+            assert_eq!(fd, 100);
+            Ok(())
+        }
+
+        fn set_vring_num(&mut self, queue_index: usize, num: u16) -> Result<()> {
+            assert_eq!(queue_index, 1);
+            assert_eq!(num, 256);
+            Ok(())
+        }
+
+        fn set_vring_addr(
+            &mut self,
+            queue_index: usize,
+            _config_data: &VringConfigData,
+        ) -> Result<()> {
+            assert_eq!(queue_index, 1);
+            Ok(())
+        }
+
+        fn set_vring_base(&mut self, queue_index: usize, base: u16) -> Result<()> {
+            assert_eq!(queue_index, 1);
+            assert_eq!(base, 2);
+            Ok(())
+        }
+
+        fn get_vring_base(&mut self, queue_index: usize) -> Result<u32> {
+            assert_eq!(queue_index, 1);
+            Ok(2)
+        }
+
+        fn set_vring_call(&mut self, queue_index: usize, _fd: &EventFd) -> Result<()> {
+            assert_eq!(queue_index, 1);
+            Ok(())
+        }
+
+        fn set_vring_kick(&mut self, queue_index: usize, _fd: &EventFd) -> Result<()> {
+            assert_eq!(queue_index, 1);
+            Ok(())
+        }
+
+        fn set_vring_err(&mut self, queue_index: usize, _fd: &EventFd) -> Result<()> {
+            assert_eq!(queue_index, 1);
+            Ok(())
+        }
+    }
+
+    #[test]
+    fn test_vring_backend_mut() {
+        let b = RwLock::new(MockBackend {});
+
+        assert_eq!(b.get_features().unwrap(), 0x1);
+        b.set_features(0x1).unwrap();
+        b.set_owner().unwrap();
+        b.reset_owner().unwrap();
+        b.set_mem_table(&[]).unwrap();
+        b.set_log_base(0x100, Some(100)).unwrap();
+        b.set_log_fd(100).unwrap();
+        b.set_vring_num(1, 256).unwrap();
+
+        let config = VringConfigData {
+            queue_max_size: 0x1000,
+            queue_size: 0x2000,
+            flags: 0x0,
+            desc_table_addr: 0x4000,
+            used_ring_addr: 0x5000,
+            avail_ring_addr: 0x6000,
+            log_addr: None,
+        };
+        b.set_vring_addr(1, &config).unwrap();
+
+        b.set_vring_base(1, 2).unwrap();
+        assert_eq!(b.get_vring_base(1).unwrap(), 2);
+
+        let eventfd = EventFd::new(0).unwrap();
+        b.set_vring_call(1, &eventfd).unwrap();
+        b.set_vring_kick(1, &eventfd).unwrap();
+        b.set_vring_err(1, &eventfd).unwrap();
+    }
+
     #[test]
     fn test_vring_config_data() {
         let mut config = VringConfigData {
diff --git a/src/vhost_kern/mod.rs b/src/vhost_kern/mod.rs
index 350e134..248cbae 100644
--- a/src/vhost_kern/mod.rs
+++ b/src/vhost_kern/mod.rs
@@ -87,20 +87,20 @@ pub trait VhostKernBackend: AsRawFd {
 impl<T: VhostKernBackend> VhostBackend for T {
     /// Set the current process as the owner of this file descriptor.
     /// This must be run before any other vhost ioctls.
-    fn set_owner(&mut self) -> Result<()> {
+    fn set_owner(&self) -> Result<()> {
         // This ioctl is called on a valid vhost fd and has its return value checked.
         let ret = unsafe { ioctl(self, VHOST_SET_OWNER()) };
         ioctl_result(ret, ())
     }
 
-    fn reset_owner(&mut self) -> Result<()> {
+    fn reset_owner(&self) -> Result<()> {
         // This ioctl is called on a valid vhost fd and has its return value checked.
         let ret = unsafe { ioctl(self, VHOST_RESET_OWNER()) };
         ioctl_result(ret, ())
     }
 
     /// Get a bitmask of supported virtio/vhost features.
-    fn get_features(&mut self) -> Result<u64> {
+    fn get_features(&self) -> Result<u64> {
         let mut avail_features: u64 = 0;
         // This ioctl is called on a valid vhost fd and has its return value checked.
         let ret = unsafe { ioctl_with_mut_ref(self, VHOST_GET_FEATURES(), &mut avail_features) };
@@ -112,14 +112,14 @@ impl<T: VhostKernBackend> VhostBackend for T {
     ///
     /// # Arguments
     /// * `features` - Bitmask of features to set.
-    fn set_features(&mut self, features: u64) -> Result<()> {
+    fn set_features(&self, features: u64) -> Result<()> {
         // This ioctl is called on a valid vhost fd and has its return value checked.
         let ret = unsafe { ioctl_with_ref(self, VHOST_SET_FEATURES(), &features) };
         ioctl_result(ret, ())
     }
 
     /// Set the guest memory mappings for vhost to use.
-    fn set_mem_table(&mut self, regions: &[VhostUserMemoryRegionInfo]) -> Result<()> {
+    fn set_mem_table(&self, regions: &[VhostUserMemoryRegionInfo]) -> Result<()> {
         if regions.is_empty() || regions.len() > VHOST_MAX_MEMORY_REGIONS {
             return Err(Error::InvalidGuestMemory);
         }
@@ -148,7 +148,7 @@ impl<T: VhostKernBackend> VhostBackend for T {
     ///
     /// # Arguments
     /// * `base` - Base address for page modification logging.
-    fn set_log_base(&mut self, base: u64, fd: Option<RawFd>) -> Result<()> {
+    fn set_log_base(&self, base: u64, fd: Option<RawFd>) -> Result<()> {
         if fd.is_some() {
             return Err(Error::LogAddress);
         }
@@ -159,7 +159,7 @@ impl<T: VhostKernBackend> VhostBackend for T {
     }
 
     /// Specify an eventfd file descriptor to signal on log write.
-    fn set_log_fd(&mut self, fd: RawFd) -> Result<()> {
+    fn set_log_fd(&self, fd: RawFd) -> Result<()> {
         // This ioctl is called on a valid vhost fd and has its return value checked.
         let val: i32 = fd;
         let ret = unsafe { ioctl_with_ref(self, VHOST_SET_LOG_FD(), &val) };
@@ -171,7 +171,7 @@ impl<T: VhostKernBackend> VhostBackend for T {
     /// # Arguments
     /// * `queue_index` - Index of the queue to set descriptor count for.
     /// * `num` - Number of descriptors in the queue.
-    fn set_vring_num(&mut self, queue_index: usize, num: u16) -> Result<()> {
+    fn set_vring_num(&self, queue_index: usize, num: u16) -> Result<()> {
         let vring_state = vhost_vring_state {
             index: queue_index as u32,
             num: u32::from(num),
@@ -187,7 +187,7 @@ impl<T: VhostKernBackend> VhostBackend for T {
     /// # Arguments
     /// * `queue_index` - Index of the queue to set addresses for.
     /// * `config_data` - Vring config data.
-    fn set_vring_addr(&mut self, queue_index: usize, config_data: &VringConfigData) -> Result<()> {
+    fn set_vring_addr(&self, queue_index: usize, config_data: &VringConfigData) -> Result<()> {
         if !self.is_valid(config_data) {
             return Err(Error::InvalidQueue);
         }
@@ -212,7 +212,7 @@ impl<T: VhostKernBackend> VhostBackend for T {
     /// # Arguments
     /// * `queue_index` - Index of the queue to modify.
     /// * `num` - Index where available descriptors start.
-    fn set_vring_base(&mut self, queue_index: usize, base: u16) -> Result<()> {
+    fn set_vring_base(&self, queue_index: usize, base: u16) -> Result<()> {
         let vring_state = vhost_vring_state {
             index: queue_index as u32,
             num: u32::from(base),
@@ -224,7 +224,7 @@ impl<T: VhostKernBackend> VhostBackend for T {
     }
 
     /// Get a bitmask of supported virtio/vhost features.
-    fn get_vring_base(&mut self, queue_index: usize) -> Result<u32> {
+    fn get_vring_base(&self, queue_index: usize) -> Result<u32> {
         let vring_state = vhost_vring_state {
             index: queue_index as u32,
             num: 0,
@@ -239,7 +239,7 @@ impl<T: VhostKernBackend> VhostBackend for T {
     /// # Arguments
     /// * `queue_index` - Index of the queue to modify.
     /// * `fd` - EventFd to trigger.
-    fn set_vring_call(&mut self, queue_index: usize, fd: &EventFd) -> Result<()> {
+    fn set_vring_call(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
         let vring_file = vhost_vring_file {
             index: queue_index as u32,
             fd: fd.as_raw_fd(),
@@ -256,7 +256,7 @@ impl<T: VhostKernBackend> VhostBackend for T {
     /// # Arguments
     /// * `queue_index` - Index of the queue to modify.
     /// * `fd` - EventFd that will be signaled from guest.
-    fn set_vring_kick(&mut self, queue_index: usize, fd: &EventFd) -> Result<()> {
+    fn set_vring_kick(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
         let vring_file = vhost_vring_file {
             index: queue_index as u32,
             fd: fd.as_raw_fd(),
@@ -272,7 +272,7 @@ impl<T: VhostKernBackend> VhostBackend for T {
     /// # Arguments
     /// * `queue_index` - Index of the queue to modify.
     /// * `fd` - EventFd that will be signaled from the backend.
-    fn set_vring_err(&mut self, queue_index: usize, fd: &EventFd) -> Result<()> {
+    fn set_vring_err(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
         let vring_file = vhost_vring_file {
             index: queue_index as u32,
             fd: fd.as_raw_fd(),
diff --git a/src/vhost_user/master.rs b/src/vhost_user/master.rs
index ffed909..2651b84 100644
--- a/src/vhost_user/master.rs
+++ b/src/vhost_user/master.rs
@@ -115,7 +115,7 @@ impl Master {
 
 impl VhostBackend for Master {
     /// Get from the underlying vhost implementation the feature bitmask.
-    fn get_features(&mut self) -> Result<u64> {
+    fn get_features(&self) -> Result<u64> {
         let mut node = self.node.lock().unwrap();
         let hdr = node.send_request_header(MasterReq::GET_FEATURES, None)?;
         let val = node.recv_reply::<VhostUserU64>(&hdr)?;
@@ -124,7 +124,7 @@ impl VhostBackend for Master {
     }
 
     /// Enable features in the underlying vhost implementation using a bitmask.
-    fn set_features(&mut self, features: u64) -> Result<()> {
+    fn set_features(&self, features: u64) -> Result<()> {
         let mut node = self.node.lock().unwrap();
         let val = VhostUserU64::new(features);
         let _ = node.send_request_with_body(MasterReq::SET_FEATURES, &val, None)?;
@@ -135,7 +135,7 @@ impl VhostBackend for Master {
     }
 
     /// Set the current Master as an owner of the session.
-    fn set_owner(&mut self) -> Result<()> {
+    fn set_owner(&self) -> Result<()> {
         // We unwrap() the return value to assert that we are not expecting threads to ever fail
         // while holding the lock.
         let mut node = self.node.lock().unwrap();
@@ -145,7 +145,7 @@ impl VhostBackend for Master {
         Ok(())
     }
 
-    fn reset_owner(&mut self) -> Result<()> {
+    fn reset_owner(&self) -> Result<()> {
         let mut node = self.node.lock().unwrap();
         let _ = node.send_request_header(MasterReq::RESET_OWNER, None)?;
         // Don't wait for ACK here because the protocol feature negotiation process hasn't been
@@ -155,7 +155,7 @@ impl VhostBackend for Master {
 
     /// Set the memory map regions on the slave so it can translate the vring
     /// addresses. In the ancillary data there is an array of file descriptors
-    fn set_mem_table(&mut self, regions: &[VhostUserMemoryRegionInfo]) -> Result<()> {
+    fn set_mem_table(&self, regions: &[VhostUserMemoryRegionInfo]) -> Result<()> {
         if regions.is_empty() || regions.len() > MAX_ATTACHED_FD_ENTRIES {
             return error_code(VhostUserError::InvalidParam);
         }
@@ -187,7 +187,7 @@ impl VhostBackend for Master {
 
     // Clippy doesn't seem to know that if let with && is still experimental
     #[allow(clippy::unnecessary_unwrap)]
-    fn set_log_base(&mut self, base: u64, fd: Option<RawFd>) -> Result<()> {
+    fn set_log_base(&self, base: u64, fd: Option<RawFd>) -> Result<()> {
         let mut node = self.node.lock().unwrap();
         let val = VhostUserU64::new(base);
 
@@ -202,7 +202,7 @@ impl VhostBackend for Master {
         Ok(())
     }
 
-    fn set_log_fd(&mut self, fd: RawFd) -> Result<()> {
+    fn set_log_fd(&self, fd: RawFd) -> Result<()> {
         let mut node = self.node.lock().unwrap();
         let fds = [fd];
         node.send_request_header(MasterReq::SET_LOG_FD, Some(&fds))?;
@@ -210,7 +210,7 @@ impl VhostBackend for Master {
     }
 
     /// Set the size of the queue.
-    fn set_vring_num(&mut self, queue_index: usize, num: u16) -> Result<()> {
+    fn set_vring_num(&self, queue_index: usize, num: u16) -> Result<()> {
         let mut node = self.node.lock().unwrap();
         if queue_index as u64 >= node.max_queue_num {
             return error_code(VhostUserError::InvalidParam);
@@ -222,7 +222,7 @@ impl VhostBackend for Master {
     }
 
     /// Sets the addresses of the different aspects of the vring.
-    fn set_vring_addr(&mut self, queue_index: usize, config_data: &VringConfigData) -> Result<()> {
+    fn set_vring_addr(&self, queue_index: usize, config_data: &VringConfigData) -> Result<()> {
         let mut node = self.node.lock().unwrap();
         if queue_index as u64 >= node.max_queue_num
             || config_data.flags & !(VhostUserVringAddrFlags::all().bits()) != 0
@@ -236,7 +236,7 @@ impl VhostBackend for Master {
     }
 
     /// Sets the base offset in the available vring.
-    fn set_vring_base(&mut self, queue_index: usize, base: u16) -> Result<()> {
+    fn set_vring_base(&self, queue_index: usize, base: u16) -> Result<()> {
         let mut node = self.node.lock().unwrap();
         if queue_index as u64 >= node.max_queue_num {
             return error_code(VhostUserError::InvalidParam);
@@ -247,7 +247,7 @@ impl VhostBackend for Master {
         node.wait_for_ack(&hdr).map_err(|e| e.into())
     }
 
-    fn get_vring_base(&mut self, queue_index: usize) -> Result<u32> {
+    fn get_vring_base(&self, queue_index: usize) -> Result<u32> {
         let mut node = self.node.lock().unwrap();
         if queue_index as u64 >= node.max_queue_num {
             return error_code(VhostUserError::InvalidParam);
@@ -263,7 +263,7 @@ impl VhostBackend for Master {
     /// Bits (0-7) of the payload contain the vring index. Bit 8 is the invalid FD flag. This flag
     /// is set when there is no file descriptor in the ancillary data. This signals that polling
     /// will be used instead of waiting for the call.
-    fn set_vring_call(&mut self, queue_index: usize, fd: &EventFd) -> Result<()> {
+    fn set_vring_call(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
         let mut node = self.node.lock().unwrap();
         if queue_index as u64 >= node.max_queue_num {
             return error_code(VhostUserError::InvalidParam);
@@ -276,7 +276,7 @@ impl VhostBackend for Master {
     /// Bits (0-7) of the payload contain the vring index. Bit 8 is the invalid FD flag. This flag
     /// is set when there is no file descriptor in the ancillary data. This signals that polling
     /// should be used instead of waiting for a kick.
-    fn set_vring_kick(&mut self, queue_index: usize, fd: &EventFd) -> Result<()> {
+    fn set_vring_kick(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
         let mut node = self.node.lock().unwrap();
         if queue_index as u64 >= node.max_queue_num {
             return error_code(VhostUserError::InvalidParam);
@@ -288,7 +288,7 @@ impl VhostBackend for Master {
     /// Set the event file descriptor to signal when error occurs.
     /// Bits (0-7) of the payload contain the vring index. Bit 8 is the invalid FD flag. This flag
     /// is set when there is no file descriptor in the ancillary data.
-    fn set_vring_err(&mut self, queue_index: usize, fd: &EventFd) -> Result<()> {
+    fn set_vring_err(&self, queue_index: usize, fd: &EventFd) -> Result<()> {
         let mut node = self.node.lock().unwrap();
         if queue_index as u64 >= node.max_queue_num {
             return error_code(VhostUserError::InvalidParam);
@@ -654,7 +654,7 @@ mod tests {
         let listener = Listener::new(UNIX_SOCKET_MASTER, true).unwrap();
         listener.set_nonblocking(true).unwrap();
 
-        let mut master = Master::connect(UNIX_SOCKET_MASTER, 1).unwrap();
+        let master = Master::connect(UNIX_SOCKET_MASTER, 1).unwrap();
         let mut slave = Endpoint::<MasterReq>::from_stream(listener.accept().unwrap().unwrap());
 
         // Send two messages continuously
@@ -692,7 +692,7 @@ mod tests {
     #[test]
     #[ignore]
     fn test_features() {
-        let (mut master, mut peer) = create_pair(UNIX_SOCKET_MASTER3);
+        let (master, mut peer) = create_pair(UNIX_SOCKET_MASTER3);
 
         master.set_owner().unwrap();
         let (hdr, rfds) = peer.recv_header().unwrap();
diff --git a/src/vhost_user/mod.rs b/src/vhost_user/mod.rs
index 48a93ff..4259d0f 100644
--- a/src/vhost_user/mod.rs
+++ b/src/vhost_user/mod.rs
@@ -203,7 +203,7 @@ mod tests {
     #[test]
     fn test_set_owner() {
         let slave_be = Arc::new(Mutex::new(DummySlaveReqHandler::new()));
-        let (mut master, mut slave) =
+        let (master, mut slave) =
             create_slave("/tmp/vhost_user_lib_unit_test_owner", slave_be.clone());
 
         assert_eq!(slave_be.lock().unwrap().owned, false);