On Wed, Feb 05, 2025 at 10:10:01AM +0100, Paolo Bonzini wrote: > Date: Wed, 5 Feb 2025 10:10:01 +0100 > From: Paolo Bonzini <pbonz...@redhat.com> > Subject: Re: [PATCH 02/10] rust: qom: add reference counting functionality > > On 2/5/25 10:13, Zhao Liu wrote: > > > > * The use of from(): > > > > > > > > let clk = bindings::qdev_init_clock_in(...) > > > > Owned::from(&*clk) > > > > > > In this case the C side wants to manage the reference that > > > qdev_init_clock_in() returns; it is dropped in > > > qdev_finalize_clocklist(). So Rust code needs to increase the > > > refcount. > > > > Pls forgive me for one more question about qdev_init_clock_in() on the C > > side. :-) > > > > qdev_init_clock_in() didn't unref `clk` after object_property_add_child(), > > so it is intentional, to make the ref count of `clk` be 2: > > * 1 count is held by clocklist until qdev_finalize_clocklist(). > > * another 1 is held by its parent via QOM Child<>. > > > > Am I understanding it correctly? > > Yes, that's more precise. In Rust it will be 3, the two above plus the > Owned<Clock>.
Thanks! > Ah wait... qdev_finalize_clocklist() is only called _after_ the Rust struct > is Drop::drop-ped, because device_finalize() is called after the subclass's > instance_finalize. > > So the result of qdev_init_clock_in(), strictly speaking, does not have to > be an Owned<Clock>. It can also be a &'device Clock; either is possible. > Would you prefer that, or do you think it's enough to add a comment? > I prefer the latter, i.e., keep current Owned<Clock> and add a comment to mention something like "Although strictly speaking, it does not have to be an Owned<Clock>. Owned<> is still worth favor to use to protect Rust code from FFI. When unsure whether to use Owned<>, then use Owned<>." -Zhao
