[PATCH 4/5] rust: migration: implement ToMigrationState for Timer

[Paolo Bonzini <pbonzini@redhat.com>]

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 |  1 -
 rust/migration/src/migratable.rs | 31 +++++++++++++++++++++++++++++++
 rust/util/src/timer.rs           | 10 +++++++++-
 3 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/rust/hw/timer/hpet/src/device.rs b/rust/hw/timer/hpet/src/device.rs
index 5bcf151a680..373ec37bbd3 100644
--- a/rust/hw/timer/hpet/src/device.rs
+++ b/rust/hw/timer/hpet/src/device.rs
@@ -250,7 +250,6 @@ const fn get_individual_route(&self) -> usize {
 }
 
 /// HPET Timer Abstraction
-#[repr(C)]
 #[derive(Debug)]
 pub struct HPETTimer {
     /// timer N index within the timer block (`HPETState`)
diff --git a/rust/migration/src/migratable.rs b/rust/migration/src/migratable.rs
index 02efe31d72c..c82a6b9a7cf 100644
--- a/rust/migration/src/migratable.rs
+++ b/rust/migration/src/migratable.rs
@@ -140,6 +140,26 @@ fn restore_migrated_state_mut(
 
 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 @@ fn restore_migrated_state(
     ) -> 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(source as u64);
+        } else {
+            self.delete();
+        }
+        Ok(())
+    }
+}
+
 impl<T: ToMigrationStateShared, const N: usize> ToMigrationStateShared for [T; N]
 where
     [T::Migrated; N]: Default,
diff --git a/rust/util/src/timer.rs b/rust/util/src/timer.rs
index 829f52d111e..4109d84c398 100644
--- a/rust/util/src/timer.rs
+++ b/rust/util/src/timer.rs
@@ -10,7 +10,8 @@
 use common::{callbacks::FnCall, Opaque};
 
 use crate::bindings::{
-    self, qemu_clock_get_ns, timer_del, timer_init_full, timer_mod, QEMUClockType,
+    self, qemu_clock_get_ns, timer_del, timer_expire_time_ns, timer_init_full, timer_mod,
+    QEMUClockType,
 };
 
 /// A safe wrapper around [`bindings::QEMUTimer`].
@@ -88,6 +89,13 @@ pub fn init_full<T, F>(
         }
     }
 
+    pub fn expire_time_ns(&self) -> Option<i64> {
+        // SAFETY: the only way to obtain a Timer safely is via methods that
+        // take a Pin<&mut Self>, therefore the timer is pinned
+        let ret = unsafe { timer_expire_time_ns(self.as_ptr()) };
+        i64::try_from(ret).ok()
+    }
+
     pub fn modify(&self, expire_time: u64) {
         // SAFETY: the only way to obtain a Timer safely is via methods that
         // take a Pin<&mut Self>, therefore the timer is pinned
-- 
2.51.1