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>

vm-virtio/src/transport/mmio.rs

@@ -2,20 +2,19 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-use std::result;
-use std::sync::atomic::{AtomicUsize, Ordering};
-use std::sync::{Arc, Mutex, RwLock};
-
-use byteorder::{ByteOrder, LittleEndian};
-use libc::EFD_NONBLOCK;
-
 use crate::transport::{VirtioTransport, NOTIFY_REG_OFFSET};
 use crate::{
     Queue, VirtioDevice, VirtioInterrupt, VirtioInterruptType, DEVICE_ACKNOWLEDGE, DEVICE_DRIVER,
     DEVICE_DRIVER_OK, DEVICE_FAILED, DEVICE_FEATURES_OK, DEVICE_INIT,
     INTERRUPT_STATUS_CONFIG_CHANGED, INTERRUPT_STATUS_USED_RING,
 };
+use arc_swap::ArcSwap;
+use byteorder::{ByteOrder, LittleEndian};
 use devices::{BusDevice, Interrupt};
+use libc::EFD_NONBLOCK;
+use std::result;
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::{Arc, Mutex};
 use vm_device::{Migratable, MigratableError, Pausable, Snapshotable};
 use vm_memory::{GuestAddress, GuestMemoryMmap};
 use vmm_sys_util::{errno::Result, eventfd::EventFd};
@@ -52,13 +51,13 @@ pub struct MmioDevice {
     config_generation: u32,
     queues: Vec<Queue>,
     queue_evts: Vec<EventFd>,
-    mem: Option<Arc<RwLock<GuestMemoryMmap>>>,
+    mem: Option<Arc<ArcSwap<GuestMemoryMmap>>>,
 }
 
 impl MmioDevice {
     /// Constructs a new MMIO transport for the given virtio device.
     pub fn new(
-        mem: Arc<RwLock<GuestMemoryMmap>>,
+        mem: Arc<ArcSwap<GuestMemoryMmap>>,
         device: Arc<Mutex<dyn VirtioDevice>>,
     ) -> Result<MmioDevice> {
         let device_clone = device.clone();
@@ -102,7 +101,7 @@ impl MmioDevice {
 
     fn are_queues_valid(&self) -> bool {
         if let Some(mem) = self.mem.as_ref() {
-            self.queues.iter().all(|q| q.is_valid(&mem.read().unwrap()))
+            self.queues.iter().all(|q| q.is_valid(mem.load().as_ref()))
         } else {
             false
         }