hypervisor, vmm: Add dynamic control of logging dirty pages

This patch extends slightly the current live-migration code path with
the ability to dynamically start and stop logging dirty-pages, which
relies on two new methods added to the `hypervisor::vm::Vm` Trait. This
patch also contains a complete implementation of the two new methods
based on `kvm` and placeholders for `mshv` in the `hypervisor` crate.

Fixes: #2858

Signed-off-by: Bo Chen <chen.bo@intel.com>

hypervisor/src/kvm/mod.rs

@@ -383,6 +383,59 @@ impl vm::Vm for KvmVm {
         Ok(())
     }
 
+    ///
+    /// Start logging dirty pages
+    ///
+    fn start_dirty_log(
+        &self,
+        slot: u32,
+        guest_phys_addr: u64,
+        memory_size: u64,
+        userspace_addr: u64,
+    ) -> vm::Result<()> {
+        let region = self.make_user_memory_region(
+            slot,
+            guest_phys_addr,
+            memory_size,
+            userspace_addr,
+            false,
+            true,
+        );
+        // Safe because guest regions are guaranteed not to overlap.
+        unsafe {
+            self.fd
+                .set_user_memory_region(region)
+                .map_err(|e| vm::HypervisorVmError::StartDirtyLog(e.into()))
+        }
+    }
+
+    ///
+    /// Stop logging dirty pages
+    ///
+    fn stop_dirty_log(
+        &self,
+        slot: u32,
+        guest_phys_addr: u64,
+        memory_size: u64,
+        userspace_addr: u64,
+    ) -> vm::Result<()> {
+        let region = self.make_user_memory_region(
+            slot,
+            guest_phys_addr,
+            memory_size,
+            userspace_addr,
+            false,
+            false,
+        );
+
+        // Safe because guest regions are guaranteed not to overlap.
+        unsafe {
+            self.fd
+                .set_user_memory_region(region)
+                .map_err(|e| vm::HypervisorVmError::StopDirtyLog(e.into()))
+        }
+    }
+
     ///
     /// Get dirty pages bitmap (one bit per page)
     ///