vmm: Add RwLock to the GuestMemoryMmap

Following the refactoring of the code allowing multiple threads to
access the same instance of the guest memory, this patch goes one step
further by adding RwLock to it. This anticipates the future need for
being able to modify the content of the guest memory at runtime.

The reasons for adding regions to an existing guest memory could be:
- Add virtio-pmem and virtio-fs regions after the guest memory was
  created.
- Support future hotplug of devices, memory, or anything that would
  require more memory at runtime.

Because most of the time, the lock will be taken as read only, using
RwLock instead of Mutex is the right approach.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/rng.rs

@@ -9,7 +9,7 @@ use std::fs::File;
 use std::io;
 use std::os::unix::io::AsRawFd;
 use std::result;
-use std::sync::Arc;
+use std::sync::{Arc, RwLock};
 use std::thread;
 
 use super::Error as DeviceError;
@@ -32,7 +32,7 @@ const KILL_EVENT: DeviceEventT = 1;
 
 struct RngEpollHandler {
     queues: Vec<Queue>,
-    mem: Arc<GuestMemoryMmap>,
+    mem: Arc<RwLock<GuestMemoryMmap>>,
     random_file: File,
     interrupt_cb: Arc<VirtioInterrupt>,
     queue_evt: EventFd,
@@ -45,14 +45,14 @@ impl RngEpollHandler {
 
         let mut used_desc_heads = [(0, 0); QUEUE_SIZE as usize];
         let mut used_count = 0;
-        for avail_desc in queue.iter(&self.mem) {
+        let mem = self.mem.read().unwrap();
+        for avail_desc in queue.iter(&mem) {
             let mut len = 0;
 
             // Drivers can only read from the random device.
             if avail_desc.is_write_only() {
                 // Fill the read with data from the random device on the host.
-                if self
-                    .mem
+                if mem
                     .read_from(
                         avail_desc.addr,
                         &mut self.random_file,
@@ -69,7 +69,7 @@ impl RngEpollHandler {
         }
 
         for &(desc_index, len) in &used_desc_heads[..used_count] {
-            queue.add_used(&self.mem, desc_index, len);
+            queue.add_used(&mem, desc_index, len);
         }
         used_count > 0
     }
@@ -237,7 +237,7 @@ impl VirtioDevice for Rng {
 
     fn activate(
         &mut self,
-        mem: Arc<GuestMemoryMmap>,
+        mem: Arc<RwLock<GuestMemoryMmap>>,
         interrupt_cb: Arc<VirtioInterrupt>,
         queues: Vec<Queue>,
         mut queue_evts: Vec<EventFd>,