Re: [PATCH 04/12] rust: timer: wrap QEMUTimer with Opaque<> and express pinning requirements

[Zhao Liu <zhao1.liu@intel.com>]

On Thu, Feb 27, 2025 at 03:22:11PM +0100, Paolo Bonzini wrote:
> Date: Thu, 27 Feb 2025 15:22:11 +0100
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: [PATCH 04/12] rust: timer: wrap QEMUTimer with Opaque<> and
>  express pinning requirements
> X-Mailer: git-send-email 2.48.1
> 
> Timers must be pinned in memory, because modify() stores a pointer to them
> in the TimerList.  To ensure this is the case, replace the separate new()
> and init_full() with a single function that returns a pinned box.  Because
> the only way to obtain a Timer is through Timer::new_full(), modify()
> knows that the timer it got is also pinned.  In the future the pinning
> requirement will be expressed through the pin_init crate instead.
> 
> Note that Timer is a bit different from other users of Opaque, in that
> it is created in Rust code rather than C code.  This is why it has to
> use the unsafe Opaque::new() function.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  meson.build                    |  7 -----
>  rust/hw/timer/hpet/src/hpet.rs | 23 ++++++++---------
>  rust/qemu-api/src/timer.rs     | 47 ++++++++++++++++++++++------------
>  3 files changed, 41 insertions(+), 36 deletions(-)

Great! LGTM,

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>

(But pls wait, I have a question below...)

> @@ -156,7 +157,7 @@ pub struct HPETTimer {
>      /// timer N index within the timer block (`HPETState`)
>      #[doc(alias = "tn")]
>      index: usize,
> -    qemu_timer: Option<Box<Timer>>,
> +    qemu_timer: Option<Pin<Box<Timer>>>,

I'm removing this Option<> wrapper in migration series. This is because
Option<> can't be treated as pointer as you mentioned in [*].

So for this reason, does this mean that VMStateField cannot accept
Option<>? I realize that all the current VMStateFlags don't seem
compatible with Option<> unless a new flag is introduced.

[*]: https://lore.kernel.org/qemu-devel/9a0389fa-765c-443b-ac2f-7c99ed862982@redhat.com/

Thanks,
Zhao