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/console.rs

@@ -10,7 +10,7 @@ use std::io;
 use std::io::Write;
 use std::os::unix::io::AsRawFd;
 use std::result;
-use std::sync::{Arc, Mutex};
+use std::sync::{Arc, Mutex, RwLock};
 use std::thread;
 
 use super::Error as DeviceError;
@@ -54,7 +54,7 @@ unsafe impl ByteValued for VirtioConsoleConfig {}
 
 struct ConsoleEpollHandler {
     queues: Vec<Queue>,
-    mem: Arc<GuestMemoryMmap>,
+    mem: Arc<RwLock<GuestMemoryMmap>>,
     interrupt_cb: Arc<VirtioInterrupt>,
     in_buffer: Arc<Mutex<VecDeque<u8>>>,
     out: Box<dyn io::Write + Send>,
@@ -80,14 +80,15 @@ impl ConsoleEpollHandler {
         let mut used_count = 0;
         let mut write_count = 0;
 
-        for avail_desc in recv_queue.iter(&self.mem) {
+        let mem = self.mem.read().unwrap();
+        for avail_desc in recv_queue.iter(&mem) {
             let len;
 
             let limit = cmp::min(write_count + avail_desc.len as u32, count as u32);
             let source_slice = in_buffer
                 .drain(write_count as usize..limit as usize)
                 .collect::<Vec<u8>>();
-            let write_result = self.mem.write_slice(&source_slice[..], avail_desc.addr);
+            let write_result = mem.write_slice(&source_slice[..], avail_desc.addr);
 
             match write_result {
                 Ok(_) => {
@@ -109,7 +110,7 @@ impl ConsoleEpollHandler {
         }
 
         for &(desc_index, len) in &used_desc_heads[..used_count] {
-            recv_queue.add_used(&self.mem, desc_index, len);
+            recv_queue.add_used(&mem, desc_index, len);
         }
         used_count > 0
     }
@@ -126,11 +127,10 @@ impl ConsoleEpollHandler {
         let mut used_desc_heads = [(0, 0); QUEUE_SIZE as usize];
         let mut used_count = 0;
 
-        for avail_desc in trans_queue.iter(&self.mem) {
+        let mem = self.mem.read().unwrap();
+        for avail_desc in trans_queue.iter(&mem) {
             let len;
-            let _ = self
-                .mem
-                .write_to(avail_desc.addr, &mut self.out, avail_desc.len as usize);
+            let _ = mem.write_to(avail_desc.addr, &mut self.out, avail_desc.len as usize);
             let _ = self.out.flush();
 
             len = avail_desc.len;
@@ -139,7 +139,7 @@ impl ConsoleEpollHandler {
         }
 
         for &(desc_index, len) in &used_desc_heads[..used_count] {
-            trans_queue.add_used(&self.mem, desc_index, len);
+            trans_queue.add_used(&mem, desc_index, len);
         }
         used_count > 0
     }
@@ -432,7 +432,7 @@ impl VirtioDevice for Console {
 
     fn activate(
         &mut self,
-        mem: Arc<GuestMemoryMmap>,
+        mem: Arc<RwLock<GuestMemoryMmap>>,
         interrupt_cb: Arc<VirtioInterrupt>,
         queues: Vec<Queue>,
         mut queue_evts: Vec<EventFd>,