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/net.rs

@@ -15,7 +15,7 @@ use std::mem;
 use std::net::Ipv4Addr;
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::result;
-use std::sync::Arc;
+use std::sync::{Arc, RwLock};
 use std::thread;
 use std::vec::Vec;
 
@@ -115,7 +115,7 @@ fn vnet_hdr_len() -> usize {
 }
 
 struct NetEpollHandler {
-    mem: Arc<GuestMemoryMmap>,
+    mem: Arc<RwLock<GuestMemoryMmap>>,
     tap: Tap,
     rx: RxVirtio,
     tx: TxVirtio,
@@ -137,7 +137,8 @@ impl NetEpollHandler {
     // if a buffer was used, and false if the frame must be deferred until a buffer
     // is made available by the driver.
     fn rx_single_frame(&mut self) -> bool {
-        let mut next_desc = self.rx.queue.iter(&self.mem).next();
+        let mem = self.mem.read().unwrap();
+        let mut next_desc = self.rx.queue.iter(&mem).next();
 
         if next_desc.is_none() {
             // Queue has no available descriptors
@@ -160,7 +161,7 @@ impl NetEpollHandler {
                     }
                     let limit = cmp::min(write_count + desc.len as usize, self.rx.bytes_read);
                     let source_slice = &self.rx.frame_buf[write_count..limit];
-                    let write_result = self.mem.write_slice(source_slice, desc.addr);
+                    let write_result = mem.write_slice(source_slice, desc.addr);
 
                     match write_result {
                         Ok(_) => {
@@ -184,9 +185,7 @@ impl NetEpollHandler {
             }
         }
 
-        self.rx
-            .queue
-            .add_used(&self.mem, head_index, write_count as u32);
+        self.rx.queue.add_used(&mem, head_index, write_count as u32);
 
         // Mark that we have at least one pending packet and we need to interrupt the guest.
         self.rx.deferred_irqs = true;
@@ -246,7 +245,8 @@ impl NetEpollHandler {
     }
 
     fn process_tx(&mut self) -> result::Result<(), DeviceError> {
-        while let Some(avail_desc) = self.tx.queue.iter(&self.mem).next() {
+        let mem = self.mem.read().unwrap();
+        while let Some(avail_desc) = self.tx.queue.iter(&mem).next() {
             let head_index = avail_desc.index;
             let mut read_count = 0;
             let mut next_desc = Some(avail_desc);
@@ -268,7 +268,7 @@ impl NetEpollHandler {
             for (desc_addr, desc_len) in self.tx.iovec.drain(..) {
                 let limit = cmp::min((read_count + desc_len) as usize, self.tx.frame_buf.len());
 
-                let read_result = self.mem.read_slice(
+                let read_result = mem.read_slice(
                     &mut self.tx.frame_buf[read_count..limit as usize],
                     desc_addr,
                 );
@@ -292,7 +292,7 @@ impl NetEpollHandler {
                 }
             };
 
-            self.tx.queue.add_used(&self.mem, head_index, 0);
+            self.tx.queue.add_used(&mem, head_index, 0);
         }
 
         Ok(())
@@ -572,7 +572,7 @@ impl VirtioDevice for Net {
 
     fn activate(
         &mut self,
-        mem: Arc<GuestMemoryMmap>,
+        mem: Arc<RwLock<GuestMemoryMmap>>,
         interrupt_cb: Arc<VirtioInterrupt>,
         mut queues: Vec<Queue>,
         mut queue_evts: Vec<EventFd>,