Re: [PATCH 6/7] rust: migration: implement ToMigrationState for Timer

[Paolo Bonzini <pbonzini@redhat.com>]

On 9/29/25 18:12, Zhao Liu wrote:
>> +    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 = unsafe { &*self.as_ptr() }.expire_time;
> 
> C side checks timer_pending(), which ensures when the timer is
> inactive, it always saves u64::MAX.
> 
> But now we save the expire_time directly in Rust. I think this would be
> possible to break the migration from Rust timer to C timer, because at
> C side, timer_get() checks whether expire_time is -1 and we can't ensure
> expire_time won't store -2 (or other unusual negative values).

I think this should work in both cases because timer_pending() checks >= 
0 and negative values are extremely far in the future.  But I'll change 
it to timer_expire_time_ns() for safety and clarity.

Paolo