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>

vmm/src/device_manager.rs

@@ -16,13 +16,14 @@ use crate::memory_manager::{Error as MemoryManagerError, MemoryManager};
 use crate::vm::VmInfo;
 #[cfg(feature = "acpi")]
 use acpi_tables::{aml, aml::Aml};
+use arc_swap::ArcSwap;
 use arch::layout;
+use arch::layout::{APIC_START, IOAPIC_SIZE, IOAPIC_START};
 use devices::{ioapic, HotPlugNotificationType};
 use kvm_bindings::kvm_irq_routing_entry;
 use kvm_ioctls::*;
 use libc::O_TMPFILE;
 use libc::TIOCGWINSZ;
-
 use net_util::Tap;
 #[cfg(feature = "pci_support")]
 use pci::{
@@ -30,20 +31,17 @@ use pci::{
     PciConfigIo, PciConfigMmio, PciDevice, PciRoot,
 };
 use qcow::{self, ImageType, QcowFile};
-
-use std::fs::{File, OpenOptions};
-use std::io::{self, sink, stdout};
-
-use arch::layout::{APIC_START, IOAPIC_SIZE, IOAPIC_START};
 use std::cmp;
 use std::collections::HashMap;
+use std::fs::{File, OpenOptions};
+use std::io::{self, sink, stdout};
 use std::os::unix::fs::OpenOptionsExt;
 use std::os::unix::io::AsRawFd;
 use std::ptr::null_mut;
 use std::result;
 #[cfg(feature = "pci_support")]
 use std::sync::Weak;
-use std::sync::{Arc, Mutex, RwLock};
+use std::sync::{Arc, Mutex};
 #[cfg(feature = "pci_support")]
 use vfio::{VfioDevice, VfioDmaMapping, VfioPciDevice, VfioPciError};
 use vm_allocator::SystemAllocator;
@@ -741,7 +739,7 @@ impl DeviceManager {
         {
             // Add a CMOS emulated device
             use vm_memory::GuestMemory;
-            let mem_size = _vm_info.memory.as_ref().read().unwrap().end_addr().0 + 1;
+            let mem_size = _vm_info.memory.load().end_addr().0 + 1;
             let mem_below_4g = std::cmp::min(arch::layout::MEM_32BIT_RESERVED_START.0, mem_size);
             let mem_above_4g = mem_size.saturating_sub(arch::layout::RAM_64BIT_START.0);
 
@@ -1402,7 +1400,7 @@ impl DeviceManager {
     #[allow(clippy::too_many_arguments)]
     fn add_virtio_pci_device(
         virtio_device: Arc<Mutex<dyn vm_virtio::VirtioDevice>>,
-        memory: &Arc<RwLock<GuestMemoryMmap>>,
+        memory: &Arc<ArcSwap<GuestMemoryMmap>>,
         address_manager: &Arc<AddressManager>,
         vm_fd: &Arc<VmFd>,
         pci: &mut PciBus,
@@ -1523,7 +1521,7 @@ impl DeviceManager {
     #[cfg(feature = "mmio_support")]
     fn add_virtio_mmio_device(
         virtio_device: Arc<Mutex<dyn vm_virtio::VirtioDevice>>,
-        memory: &Arc<RwLock<GuestMemoryMmap>>,
+        memory: &Arc<ArcSwap<GuestMemoryMmap>>,
         address_manager: &Arc<AddressManager>,
         vm_fd: &Arc<VmFd>,
         interrupt_info: &InterruptInfo,