Re: [RFC PATCH 3/9] rust: vmstate: add varray support to vmstate_of!

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

On Tue, Dec 31, 2024 at 01:23:30AM +0100, Paolo Bonzini wrote:
> Date: Tue, 31 Dec 2024 01:23:30 +0100
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: [RFC PATCH 3/9] rust: vmstate: add varray support to vmstate_of!
> X-Mailer: git-send-email 2.47.1
> 
> Untested...
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  rust/qemu-api/src/vmstate.rs | 45 +++++++++++++++++++++++++++++++++---
>  1 file changed, 42 insertions(+), 3 deletions(-)
> 
> diff --git a/rust/qemu-api/src/vmstate.rs b/rust/qemu-api/src/vmstate.rs
> index e20f27b172b..079c19c687b 100644
> --- a/rust/qemu-api/src/vmstate.rs
> +++ b/rust/qemu-api/src/vmstate.rs
> @@ -69,6 +69,15 @@ pub unsafe trait VMState {
>      /// The base contents of a `VMStateField` (minus the name and offset) for
>      /// the type that is implementing the trait.
>      const BASE: VMStateField;
> +
> +    /// A flag that is added to another field's `VMStateField` to specify the
> +    /// length's type in a variable-sized array.  If this is not a supported
> +    /// type for the length (i.e. if it is not `u8`, `u16`, `u32`), using it
> +    /// in a call to [`vmstate_of!`](crate::vmstate_of) will cause a
> +    /// compile-time error.
> +    const VARRAY_FLAG: VMStateFlags = {
> +        panic!("invalid type for variable-sized array");
> +    };
>  }
>  
>  /// Internal utility function to retrieve a type's `VMStateField`;
> @@ -77,6 +86,13 @@ pub const fn vmstate_base<T: VMState>(_: PhantomData<T>) -> VMStateField {
>      T::BASE
>  }
>  
> +/// Internal utility function to retrieve a type's `VMStateFlags` when it
> +/// is used as the element count of a `VMSTATE_VARRAY`; used by
> +/// [`vmstate_of!`](crate::vmstate_of).
> +pub const fn vmstate_varray_flag<T: VMState>(_: PhantomData<T>) -> VMStateField {
> +    T::BASE
> +}

a copy issue:

pub const fn vmstate_varray_flag<T: VMState>(_: PhantomData<T>) -> VMStateFlags {
    T::VARRAY_FLAG
}

> +
>  /// Return the `VMStateField` for a field of a struct.  The field must be
>  /// visible in the current scope.
>  ///
> @@ -84,18 +100,24 @@ pub const fn vmstate_base<T: VMState>(_: PhantomData<T>) -> VMStateField {
>  /// for them.
>  #[macro_export]
>  macro_rules! vmstate_of {
> -    ($struct_name:ty, $field_name:ident $(,)?) => {
> +    ($struct_name:ty, $field_name:ident $([0 .. $num:ident $(* $factor:expr)?])? $(,)?) => {

Compared to something like "[num]", the format "[0 .. num]" is more
elegant, so I also support this style.

>          $crate::bindings::VMStateField {
>              name: ::core::concat!(::core::stringify!($field_name), "\0")
>                  .as_bytes()
>                  .as_ptr() as *const ::std::os::raw::c_char,
>              offset: $crate::offset_of!($struct_name, $field_name),
> -            // Compute most of the VMStateField from the type of the field.
> +            $(.num_offset: $crate::offset_of!($struct_name, $num),)?
> +            // The calls to `call_func_with_field!` are the magic that
> +            // computes most of the VMStateField from the type of the field.
>              ..$crate::call_func_with_field!(
>                  $crate::vmstate::vmstate_base,
>                  $struct_name,
>                  $field_name
> -            )
> +            )$(.with_varray_flag($crate::call_func_with_field!(
> +                    $crate::vmstate::vmstate_varray_flag,
> +                    $struct_name,
> +                    $num))
> +               $(.with_varray_multiply($factor))?)?
>          }
>      };
>  }
> @@ -130,6 +152,22 @@ pub const fn with_pointer_flag(mut self) -> Self {
>          self.flags = VMStateFlags(self.flags.0 | VMStateFlags::VMS_POINTER.0);
>          self
>      }
> +
> +    #[must_use]
> +    pub const fn with_varray_flag<T: VMState>(mut self, flag: VMStateFlags) -> VMStateField {
> +        assert!((self.flags.0 & VMStateFlags::VMS_ARRAY.0) != 0);

I understand you checked VMS_ARRAY here since [T; N] has this array
flag.

What if a Rust device just store a pointer to the array? If we allow
this use, then it seems also possible to set varray flags...Then what
about dropping this limitation?

However, I also doube that pointer usage is bad; we should always use
Vec.

> +        self.flags = VMStateFlags(self.flags.0 & !VMStateFlags::VMS_ARRAY.0);
> +        self.flags = VMStateFlags(self.flags.0 | flag.0);
> +        self
> +    }
> +
> +    #[must_use]
> +    pub const fn with_varray_multiply(mut self, num: u32) -> VMStateField {
> +        assert!(num <= 0x7FFF_FFFFu32);

Similarly, what about using "assert!(num <= i32::MAX as u32);"?

> +        self.flags = VMStateFlags(self.flags.0 | VMStateFlags::VMS_MULTIPLY_ELEMENTS.0);
> +        self.num = num as i32;
> +        self
> +    }
>  }
>  
>  // Transparent wrappers: just use the internal type
> @@ -141,6 +179,7 @@ unsafe impl<$base> VMState for $type where $base: VMState $($where)* {
>                  size: mem::size_of::<$type>(),
>                  ..<$base as VMState>::BASE
>              };
> +            const VARRAY_FLAG: VMStateFlags = <$base as VMState>::VARRAY_FLAG;
>          }
>      };
>  }
> -- 
> 2.47.1
>