vm-virtio, vmm, vfio: Store GuestMemoryMmap in an Arc<ArcSwap<T>>

This allows us to change the memory map that is being used by the
devices via an atomic swap (by replacing the map with another one). The
ArcSwap provides the mechanism for atomically swapping from to another
whilst still giving good read performace. It is inside an Arc so that we
can use a single ArcSwap for all users.

Not covered by this change is replacing the GuestMemoryMmap itself.

This change also removes some vertical whitespace from use blocks in the
files that this commit also changed. Vertical whitespace was being used
inconsistently and broke rustfmt's behaviour of ordering the imports as
it would only do it within the block.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vfio/src/lib.rs

@@ -5,6 +5,7 @@
 
 //#![deny(missing_docs)]
 //! Virtual Function I/O (VFIO) API
+extern crate arc_swap;
 extern crate byteorder;
 extern crate devices;
 extern crate kvm_bindings;

vfio/src/vfio_device.rs

@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 OR BSD-3-Clause
 //
 use crate::vec_with_array_field;
+use arc_swap::ArcSwap;
 use byteorder::{ByteOrder, LittleEndian};
 use kvm_ioctls::*;
 use std::collections::HashMap;
@@ -16,7 +17,7 @@ use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
 use std::os::unix::prelude::FileExt;
 use std::path::{Path, PathBuf};
 use std::result;
-use std::sync::{Arc, RwLock};
+use std::sync::Arc;
 use std::u32;
 use vfio_bindings::bindings::vfio::*;
 use vfio_ioctls::*;
@@ -514,11 +515,11 @@ impl VfioDeviceInfo {
 /// associated with a specific VFIO container.
 pub struct VfioDmaMapping {
     container: Arc<VfioContainer>,
-    memory: Arc<RwLock<GuestMemoryMmap>>,
+    memory: Arc<ArcSwap<GuestMemoryMmap>>,
 }
 
 impl VfioDmaMapping {
-    pub fn new(container: Arc<VfioContainer>, memory: Arc<RwLock<GuestMemoryMmap>>) -> Self {
+    pub fn new(container: Arc<VfioContainer>, memory: Arc<ArcSwap<GuestMemoryMmap>>) -> Self {
         VfioDmaMapping { container, memory }
     }
 }
@@ -526,7 +527,7 @@ impl VfioDmaMapping {
 impl ExternalDmaMapping for VfioDmaMapping {
     fn map(&self, iova: u64, gpa: u64, size: u64) -> result::Result<(), io::Error> {
         let user_addr = if let Some(addr) = get_host_address_range(
-            &self.memory.read().unwrap(),
+            &self.memory.load(),
             GuestAddress(gpa),
             size.try_into().unwrap(),
         ) {
@@ -577,7 +578,7 @@ pub struct VfioDevice {
     group: VfioGroup,
     regions: Vec<VfioRegion>,
     irqs: HashMap<u32, VfioIrq>,
-    mem: Arc<RwLock<GuestMemoryMmap>>,
+    mem: Arc<ArcSwap<GuestMemoryMmap>>,
     iommu_attached: bool,
 }
 
@@ -588,7 +589,7 @@ impl VfioDevice {
     pub fn new(
         sysfspath: &Path,
         device_fd: Arc<DeviceFd>,
-        mem: Arc<RwLock<GuestMemoryMmap>>,
+        mem: Arc<ArcSwap<GuestMemoryMmap>>,
         iommu_attached: bool,
     ) -> Result<Self> {
         let uuid_path: PathBuf = [sysfspath, Path::new("iommu_group")].iter().collect();
@@ -845,7 +846,7 @@ impl VfioDevice {
     /// then vfio kernel driver could access guest memory from gfn
     pub fn setup_dma_map(&self) -> Result<()> {
         if !self.iommu_attached {
-            self.mem.read().unwrap().with_regions(|_index, region| {
+            self.mem.load().with_regions(|_index, region| {
                 self.vfio_dma_map(
                     region.start_addr().raw_value(),
                     region.len() as u64,
@@ -860,7 +861,7 @@ impl VfioDevice {
     /// then vfio kernel driver couldn't access this guest memory
     pub fn unset_dma_map(&self) -> Result<()> {
         if !self.iommu_attached {
-            self.mem.read().unwrap().with_regions(|_index, region| {
+            self.mem.load().with_regions(|_index, region| {
                 self.vfio_dma_unmap(region.start_addr().raw_value(), region.len() as u64)
             })?;
         }