rust: migration: implement ToMigrationState for Timer

Timer is a complex struct, allow adding it to a struct that
uses #[derive(ToMigrationState)]; similar to vmstate_timer, only
the expiration time has to be preserved.

In fact, because it is thread-safe, ToMigrationStateShared can
also be implemented without needing a cell or mutex that wraps
the timer.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/hw/timer/hpet/src/device.rs

@@ -250,7 +250,6 @@ impl HPETTimerRegisters {
 }
 
 /// HPET Timer Abstraction
-#[repr(C)]
 #[derive(Debug)]
 pub struct HPETTimer {
     /// timer N index within the timer block (`HPETState`)

rust/migration/src/migratable.rs

@@ -140,6 +140,26 @@ macro_rules! impl_for_primitive {
 
 impl_for_primitive!(u8, u16, u32, u64, i8, i16, i32, i64, bool);
 
+impl ToMigrationState for util::timer::Timer {
+    type Migrated = i64;
+
+    fn snapshot_migration_state(&self, target: &mut i64) -> Result<(), InvalidError> {
+        // SAFETY: as_ptr() is unsafe to ensure that the caller reasons about
+        // the pinning of the data inside the Opaque<>.  Here all we do is
+        // access a field.
+        *target = self.expire_time_ns().unwrap_or(-1);
+        Ok(())
+    }
+
+    fn restore_migrated_state_mut(
+        &mut self,
+        source: Self::Migrated,
+        version_id: u8,
+    ) -> Result<(), InvalidError> {
+        self.restore_migrated_state(source, version_id)
+    }
+}
+
 impl<T: ToMigrationState, const N: usize> ToMigrationState for [T; N]
 where
     [T::Migrated; N]: Default,
@@ -237,6 +257,17 @@ pub trait ToMigrationStateShared: ToMigrationState {
     ) -> Result<(), InvalidError>;
 }
 
+impl ToMigrationStateShared for util::timer::Timer {
+    fn restore_migrated_state(&self, source: i64, _version_id: u8) -> Result<(), InvalidError> {
+        if source >= 0 {
+            self.modify_ns(source as u64);
+        } else {
+            self.delete();
+        }
+        Ok(())
+    }
+}
+
 impl<T: ToMigrationStateShared, const N: usize> ToMigrationStateShared for [T; N]
 where
     [T::Migrated; N]: Default,