Re: [PATCH v2] rust/qemu-macros: Convert bit value to u8 within #[property]

[Manos Pitsidianakis <manos.pitsidianakis@linaro.org>]

On Fri, 24 Oct 2025 07:13, Zhao Liu <zhao1.liu@intel.com> wrote:
>For bit property, make the type conversion within the #[property] macro
>so that users do not need to handle the conversion.
>
>Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
>Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
>---
>Changes Since v2:
> - Check #field_ty::BITS instead of u8::MAX.
> - Update test cases.
>---


LGTM, some questions...

> rust/hw/timer/hpet/src/device.rs |  2 +-
> rust/qemu-macros/src/lib.rs      | 23 ++++++++++++++++++-----
> rust/qemu-macros/src/tests.rs    | 15 ++++++++++++---
> 3 files changed, 31 insertions(+), 9 deletions(-)
>
>diff --git a/rust/hw/timer/hpet/src/device.rs b/rust/hw/timer/hpet/src/device.rs
>index 86638c076666..23f2eefd1cd9 100644
>--- a/rust/hw/timer/hpet/src/device.rs
>+++ b/rust/hw/timer/hpet/src/device.rs
>@@ -539,7 +539,7 @@ pub struct HPETState {
>     // Internal state
>     /// Capabilities that QEMU HPET supports.
>     /// bit 0: MSI (or FSB) support.
>-    #[property(rename = "msi", bit = HPET_FLAG_MSI_SUPPORT_SHIFT as u8, default = false)]
>+    #[property(rename = "msi", bit = HPET_FLAG_MSI_SUPPORT_SHIFT, default = false)]
>     flags: u32,
> 
>     /// Offset of main counter relative to qemu clock.
>diff --git a/rust/qemu-macros/src/lib.rs b/rust/qemu-macros/src/lib.rs
>index 50239f228be2..ee417bb4b4ef 100644
>--- a/rust/qemu-macros/src/lib.rs
>+++ b/rust/qemu-macros/src/lib.rs
>@@ -262,12 +262,25 @@ macro_rules! str_to_c_str {
>             },
>         )?;
>         let field_ty = field.ty.clone();
>-        let qdev_prop = if bitnr.is_none() {
>-            quote! { <#field_ty as ::hwcore::QDevProp>::BASE_INFO }
>+        let (qdev_prop, bitval) = if let Some(bitval) = bitnr {
>+            (
>+                quote! { <#field_ty as ::hwcore::QDevProp>::BIT_INFO },
>+                quote! {
>+                    {
>+                        const {
>+                            assert!(#bitval >= 0 && #bitval < #field_ty::BITS as _,
>+                                    "bit number exceeds type bits 
>range");

Const panic messages cannot use formatting parameters yet, but.
Can we interpolate the type (e.g u32) in the compile-time panic message? 
Not important but would make the error message friendlier.


Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>

>+                        }
>+                        #bitval as u8
>+                    }
>+                },
>+            )
>         } else {
>-            quote! { <#field_ty as ::hwcore::QDevProp>::BIT_INFO }
>+            (
>+                quote! { <#field_ty as ::hwcore::QDevProp>::BASE_INFO },
>+                quote! { 0 },
>+            )
>         };
>-        let bitnr = bitnr.unwrap_or(syn::Expr::Verbatim(quote! { 0 }));
>         let set_default = defval.is_some();
>         let defval = defval.unwrap_or(syn::Expr::Verbatim(quote! { 0 }));
>         properties_expanded.push(quote! {
>@@ -275,7 +288,7 @@ macro_rules! str_to_c_str {
>                 name: ::std::ffi::CStr::as_ptr(#prop_name),
>                 info: #qdev_prop,
>                 offset: ::core::mem::offset_of!(#name, #field_name) as isize,
>-                bitnr: #bitnr,
>+                bitnr: #bitval,
>                 set_default: #set_default,
>                 defval: ::hwcore::bindings::Property__bindgen_ty_1 { u: #defval as u64 },
>                 ..::common::Zeroable::ZERO
>diff --git a/rust/qemu-macros/src/tests.rs b/rust/qemu-macros/src/tests.rs
>index 65691412ff57..b65cf656fa36 100644
>--- a/rust/qemu-macros/src/tests.rs
>+++ b/rust/qemu-macros/src/tests.rs
>@@ -179,7 +179,10 @@ unsafe impl ::hwcore::DevicePropertiesImpl for DummyState {
>                         name: ::std::ffi::CStr::as_ptr(c"flags"),
>                         info: <u32 as ::hwcore::QDevProp>::BIT_INFO,
>                         offset: ::core::mem::offset_of!(DummyState, flags) as isize,
>-                        bitnr: 3,
>+                        bitnr : {
>+                            const { assert!(3 >= 0 && 3 < u32::BITS as _ , "bit number exceeds type bits range"); }
>+                            3 as u8
>+                        },
>                         set_default: false,
>                         defval: ::hwcore::bindings::Property__bindgen_ty_1 { u: 0 as u64 },
>                         ..::common::Zeroable::ZERO
>@@ -207,7 +210,10 @@ unsafe impl ::hwcore::DevicePropertiesImpl for DummyState {
>                         name: ::std::ffi::CStr::as_ptr(c"flags"),
>                         info: <u32 as ::hwcore::QDevProp>::BIT_INFO,
>                         offset: ::core::mem::offset_of!(DummyState, flags) as isize,
>-                        bitnr: 3,
>+                        bitnr : {
>+                            const { assert!(3 >= 0 && 3 < u32::BITS as _ , "bit number exceeds type bits range"); }
>+                            3 as u8
>+                        },
>                         set_default: true,
>                         defval: ::hwcore::bindings::Property__bindgen_ty_1 { u: true as u64 },
>                         ..::common::Zeroable::ZERO
>@@ -235,7 +241,10 @@ unsafe impl ::hwcore::DevicePropertiesImpl for DummyState {
>                         name: ::std::ffi::CStr::as_ptr(c"msi"),
>                         info: <u64 as ::hwcore::QDevProp>::BIT_INFO,
>                         offset: ::core::mem::offset_of!(DummyState, flags) as isize,
>-                        bitnr: 3,
>+                        bitnr : {
>+                            const { assert!(3 >= 0 && 3 < u64::BITS as _ , "bit number exceeds type bits range"); }
>+                            3 as u8
>+                        },
>                         set_default: true,
>                         defval: ::hwcore::bindings::Property__bindgen_ty_1 { u: false as u64 },
>                         ..::common::Zeroable::ZERO
>-- 
>2.34.1
>