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/pmem.rs

@@ -15,7 +15,7 @@ use std::io::{self, Write};
 use std::mem::size_of;
 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;
@@ -154,7 +154,7 @@ impl Request {
 
 struct PmemEpollHandler {
     queue: Queue,
-    mem: Arc<GuestMemoryMmap>,
+    mem: Arc<RwLock<GuestMemoryMmap>>,
     disk: File,
     interrupt_cb: Arc<VirtioInterrupt>,
     queue_evt: EventFd,
@@ -165,8 +165,9 @@ impl PmemEpollHandler {
     fn process_queue(&mut self) -> bool {
         let mut used_desc_heads = [(0, 0); QUEUE_SIZE as usize];
         let mut used_count = 0;
-        for avail_desc in self.queue.iter(&self.mem) {
-            let len = match Request::parse(&avail_desc, &self.mem) {
+        let mem = self.mem.read().unwrap();
+        for avail_desc in self.queue.iter(&mem) {
+            let len = match Request::parse(&avail_desc, &mem) {
                 Ok(ref req) if (req.type_ == RequestType::Flush) => {
                     let status_code = match self.disk.sync_all() {
                         Ok(()) => VIRTIO_PMEM_RESP_TYPE_OK,
@@ -177,7 +178,7 @@ impl PmemEpollHandler {
                     };
 
                     let resp = VirtioPmemResp { ret: status_code };
-                    match self.mem.write_obj(resp, req.status_addr) {
+                    match mem.write_obj(resp, req.status_addr) {
                         Ok(_) => size_of::<VirtioPmemResp>() as u32,
                         Err(e) => {
                             error!("bad guest memory address: {}", e);
@@ -201,7 +202,7 @@ impl PmemEpollHandler {
         }
 
         for &(desc_index, len) in &used_desc_heads[..used_count] {
-            self.queue.add_used(&self.mem, desc_index, len);
+            self.queue.add_used(&mem, desc_index, len);
         }
         used_count > 0
     }
@@ -382,7 +383,7 @@ impl VirtioDevice for Pmem {
 
     fn activate(
         &mut self,
-        mem: Arc<GuestMemoryMmap>,
+        mem: Arc<RwLock<GuestMemoryMmap>>,
         interrupt_cb: Arc<VirtioInterrupt>,
         mut queues: Vec<Queue>,
         mut queue_evts: Vec<EventFd>,