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/console.rs

@@ -1,6 +1,13 @@
 // Copyright 2019 Intel Corporation. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
+use super::Error as DeviceError;
+use super::{
+    ActivateError, ActivateResult, DeviceEventT, Queue, VirtioDevice, VirtioDeviceType,
+    VirtioInterruptType, VIRTIO_F_IOMMU_PLATFORM, VIRTIO_F_VERSION_1,
+};
+use crate::VirtioInterrupt;
+use arc_swap::ArcSwap;
 use epoll;
 use libc::EFD_NONBLOCK;
 use std;
@@ -11,16 +18,9 @@ use std::io::Write;
 use std::ops::DerefMut;
 use std::os::unix::io::AsRawFd;
 use std::result;
-use std::sync::{Arc, Mutex, RwLock};
-use std::thread;
-
-use super::Error as DeviceError;
-use super::{
-    ActivateError, ActivateResult, DeviceEventT, Queue, VirtioDevice, VirtioDeviceType,
-    VirtioInterruptType, VIRTIO_F_IOMMU_PLATFORM, VIRTIO_F_VERSION_1,
-};
-use crate::VirtioInterrupt;
 use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
+use std::sync::{Arc, Mutex};
+use std::thread;
 use vm_device::{Migratable, MigratableError, Pausable, Snapshotable};
 use vm_memory::{ByteValued, Bytes, GuestMemoryMmap};
 use vmm_sys_util::eventfd::EventFd;
@@ -58,7 +58,7 @@ unsafe impl ByteValued for VirtioConsoleConfig {}
 
 struct ConsoleEpollHandler {
     queues: Vec<Queue>,
-    mem: Arc<RwLock<GuestMemoryMmap>>,
+    mem: Arc<ArcSwap<GuestMemoryMmap>>,
     interrupt_cb: Arc<VirtioInterrupt>,
     in_buffer: Arc<Mutex<VecDeque<u8>>>,
     out: Arc<Mutex<Box<dyn io::Write + Send + Sync + 'static>>>,
@@ -85,7 +85,7 @@ impl ConsoleEpollHandler {
         let mut used_count = 0;
         let mut write_count = 0;
 
-        let mem = self.mem.read().unwrap();
+        let mem = self.mem.load();
         for avail_desc in recv_queue.iter(&mem) {
             let len;
 
@@ -132,7 +132,7 @@ impl ConsoleEpollHandler {
         let mut used_desc_heads = [(0, 0); QUEUE_SIZE as usize];
         let mut used_count = 0;
 
-        let mem = self.mem.read().unwrap();
+        let mem = self.mem.load();
         for avail_desc in trans_queue.iter(&mem) {
             let len;
             let mut out = self.out.lock().unwrap();
@@ -473,7 +473,7 @@ impl VirtioDevice for Console {
 
     fn activate(
         &mut self,
-        mem: Arc<RwLock<GuestMemoryMmap>>,
+        mem: Arc<ArcSwap<GuestMemoryMmap>>,
         interrupt_cb: Arc<VirtioInterrupt>,
         queues: Vec<Queue>,
         mut queue_evts: Vec<EventFd>,