Merge tag 'for-upstream' of https://gitlab.com/bonzini/qemu into staging

* rust: miscellaneous changes
* target/i386: small code generation improvements
* target/i386: various cleanups and fixes
* cpu: remove env->nr_cores

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# =xIQ4
# -----END PGP SIGNATURE-----
# gpg: Signature made Fri 10 Jan 2025 17:34:48 EST
# gpg:                using RSA key F13338574B662389866C7682BFFBD25F78C7AE83
# gpg:                issuer "pbonzini@redhat.com"
# gpg: Good signature from "Paolo Bonzini <bonzini@gnu.org>" [full]
# gpg:                 aka "Paolo Bonzini <pbonzini@redhat.com>" [full]
# Primary key fingerprint: 46F5 9FBD 57D6 12E7 BFD4  E2F7 7E15 100C CD36 69B1
#      Subkey fingerprint: F133 3857 4B66 2389 866C  7682 BFFB D25F 78C7 AE83

* tag 'for-upstream' of https://gitlab.com/bonzini/qemu: (38 commits)
  i386/cpu: Set and track CPUID_EXT3_CMP_LEG in env->features[FEAT_8000_0001_ECX]
  i386/cpu: Set up CPUID_HT in x86_cpu_expand_features() instead of cpu_x86_cpuid()
  cpu: Remove nr_cores from struct CPUState
  i386/cpu: Hoist check of CPUID_EXT3_TOPOEXT against threads_per_core
  i386/cpu: Track a X86CPUTopoInfo directly in CPUX86State
  i386/topology: Introduce helpers for various topology info of different level
  i386/topology: Update the comment of x86_apicid_from_topo_ids()
  i386/cpu: Drop cores_per_pkg in cpu_x86_cpuid()
  i386/cpu: Drop the variable smp_cores and smp_threads in x86_cpu_pre_plug()
  i386/cpu: Extract a common fucntion to setup value of MSR_CORE_THREAD_COUNT
  target/i386/kvm: Replace ARRAY_SIZE(msr_handlers) with KVM_MSR_FILTER_MAX_RANGES
  target/i386/kvm: Clean up error handling in kvm_arch_init()
  target/i386/kvm: Return -1 when kvm_msr_energy_thread_init() fails
  target/i386/kvm: Clean up return values of MSR filter related functions
  target/i386/confidential-guest: Fix comment of x86_confidential_guest_kvm_type()
  target/i386/kvm: Drop workaround for KVM_X86_DISABLE_EXITS_HTL typo
  target/i386/kvm: Only save/load kvmclock MSRs when kvmclock enabled
  target/i386/kvm: Remove local MSR_KVM_WALL_CLOCK and MSR_KVM_SYSTEM_TIME definitions
  target/i386/kvm: Add feature bit definitions for KVM CPUID
  i386/cpu: Mark avx10_version filtered when prefix is NULL
  ...

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

12 files changed, 365 insertions, 129 deletions

diff --git a/rust/hw/char/pl011/src/device.rs b/rust/hw/char/pl011/src/device.rs
index 18cc122..994c2fc 100644
--- a/rust/hw/char/pl011/src/device.rs
+++ b/rust/hw/char/pl011/src/device.rs
@@ -5,7 +5,7 @@
 use core::ptr::{addr_of_mut, NonNull};
 use std::{
     ffi::CStr,
-    os::raw::{c_int, c_uchar, c_uint, c_void},
+    os::raw::{c_int, c_uint, c_void},
 };
 
 use qemu_api::{
@@ -14,7 +14,7 @@ use qemu_api::{
     irq::InterruptSource,
     prelude::*,
     qdev::DeviceImpl,
-    qom::ObjectImpl,
+    qom::{ClassInitImpl, ObjectImpl, ParentField},
 };
 
 use crate::{
@@ -33,27 +33,20 @@ const FBRD_MASK: u32 = 0x3f;
 /// QEMU sourced constant.
 pub const PL011_FIFO_DEPTH: u32 = 16;
 
-#[derive(Clone, Copy, Debug)]
-enum DeviceId {
-    #[allow(dead_code)]
-    Arm = 0,
-    Luminary,
-}
+#[derive(Clone, Copy)]
+struct DeviceId(&'static [u8; 8]);
 
 impl std::ops::Index<hwaddr> for DeviceId {
-    type Output = c_uchar;
+    type Output = u8;
 
     fn index(&self, idx: hwaddr) -> &Self::Output {
-        match self {
-            Self::Arm => &Self::PL011_ID_ARM[idx as usize],
-            Self::Luminary => &Self::PL011_ID_LUMINARY[idx as usize],
-        }
+        &self.0[idx as usize]
     }
 }
 
 impl DeviceId {
-    const PL011_ID_ARM: [c_uchar; 8] = [0x11, 0x10, 0x14, 0x00, 0x0d, 0xf0, 0x05, 0xb1];
-    const PL011_ID_LUMINARY: [c_uchar; 8] = [0x11, 0x00, 0x18, 0x01, 0x0d, 0xf0, 0x05, 0xb1];
+    const ARM: Self = Self(&[0x11, 0x10, 0x14, 0x00, 0x0d, 0xf0, 0x05, 0xb1]);
+    const LUMINARY: Self = Self(&[0x11, 0x00, 0x18, 0x01, 0x0d, 0xf0, 0x05, 0xb1]);
 }
 
 // FIFOs use 32-bit indices instead of usize, for compatibility with
@@ -86,7 +79,7 @@ impl std::ops::Index<u32> for Fifo {
 #[derive(Debug, qemu_api_macros::Object, qemu_api_macros::offsets)]
 /// PL011 Device Model in QEMU
 pub struct PL011State {
-    pub parent_obj: SysBusDevice,
+    pub parent_obj: ParentField<SysBusDevice>,
     pub iomem: MemoryRegion,
     #[doc(alias = "fr")]
     pub flags: registers::Flags,
@@ -126,21 +119,33 @@ pub struct PL011State {
     pub clock: NonNull<Clock>,
     #[doc(alias = "migrate_clk")]
     pub migrate_clock: bool,
-    /// The byte string that identifies the device.
-    device_id: DeviceId,
 }
 
 qom_isa!(PL011State : SysBusDevice, DeviceState, Object);
 
+pub struct PL011Class {
+    parent_class: <SysBusDevice as ObjectType>::Class,
+    /// The byte string that identifies the device.
+    device_id: DeviceId,
+}
+
 unsafe impl ObjectType for PL011State {
-    type Class = <SysBusDevice as ObjectType>::Class;
+    type Class = PL011Class;
     const TYPE_NAME: &'static CStr = crate::TYPE_PL011;
 }
 
+impl ClassInitImpl<PL011Class> for PL011State {
+    fn class_init(klass: &mut PL011Class) {
+        klass.device_id = DeviceId::ARM;
+        <Self as ClassInitImpl<SysBusDeviceClass>>::class_init(&mut klass.parent_class);
+    }
+}
+
 impl ObjectImpl for PL011State {
     type ParentType = SysBusDevice;
 
     const INSTANCE_INIT: Option<unsafe fn(&mut Self)> = Some(Self::init);
+    const INSTANCE_POST_INIT: Option<fn(&Self)> = Some(Self::post_init);
 }
 
 impl DeviceImpl for PL011State {
@@ -179,14 +184,6 @@ impl PL011State {
                 Self::TYPE_NAME.as_ptr(),
                 0x1000,
             );
-
-            let sbd: &mut SysBusDevice = self.upcast_mut();
-            sysbus_init_mmio(sbd, addr_of_mut!(self.iomem));
-        }
-
-        for irq in self.interrupts.iter() {
-            let sbd: &SysBusDevice = self.upcast();
-            sbd.init_irq(irq);
         }
 
         // SAFETY:
@@ -209,12 +206,20 @@ impl PL011State {
         }
     }
 
+    fn post_init(&self) {
+        self.init_mmio(&self.iomem);
+        for irq in self.interrupts.iter() {
+            self.init_irq(irq);
+        }
+    }
+
     pub fn read(&mut self, offset: hwaddr, _size: c_uint) -> std::ops::ControlFlow<u64, u64> {
         use RegisterOffset::*;
 
         let value = match RegisterOffset::try_from(offset) {
             Err(v) if (0x3f8..0x400).contains(&(v >> 2)) => {
-                u32::from(self.device_id[(offset - 0xfe0) >> 2])
+                let device_id = self.get_class().device_id;
+                u32::from(device_id[(offset - 0xfe0) >> 2])
             }
             Err(_) => {
                 // qemu_log_mask(LOG_GUEST_ERROR, "pl011_read: Bad offset 0x%x\n", (int)offset);
@@ -645,19 +650,13 @@ pub unsafe extern "C" fn pl011_create(
 #[derive(Debug, qemu_api_macros::Object)]
 /// PL011 Luminary device model.
 pub struct PL011Luminary {
-    parent_obj: PL011State,
+    parent_obj: ParentField<PL011State>,
 }
 
-impl PL011Luminary {
-    /// Initializes a pre-allocated, unitialized instance of `PL011Luminary`.
-    ///
-    /// # Safety
-    ///
-    /// We expect the FFI user of this function to pass a valid pointer, that
-    /// has the same size as [`PL011Luminary`]. We also expect the device is
-    /// readable/writeable from one thread at any time.
-    unsafe fn init(&mut self) {
-        self.parent_obj.device_id = DeviceId::Luminary;
+impl ClassInitImpl<PL011Class> for PL011Luminary {
+    fn class_init(klass: &mut PL011Class) {
+        klass.device_id = DeviceId::LUMINARY;
+        <Self as ClassInitImpl<SysBusDeviceClass>>::class_init(&mut klass.parent_class);
     }
 }
 
@@ -670,8 +669,6 @@ unsafe impl ObjectType for PL011Luminary {
 
 impl ObjectImpl for PL011Luminary {
     type ParentType = PL011State;
-
-    const INSTANCE_INIT: Option<unsafe fn(&mut Self)> = Some(Self::init);
 }
 
 impl DeviceImpl for PL011Luminary {}
diff --git a/rust/hw/char/pl011/src/lib.rs b/rust/hw/char/pl011/src/lib.rs
index 69064d6..0a89d39 100644
--- a/rust/hw/char/pl011/src/lib.rs
+++ b/rust/hw/char/pl011/src/lib.rs
@@ -45,7 +45,7 @@ pub const TYPE_PL011_LUMINARY: &::std::ffi::CStr = c_str!("pl011_luminary");
 #[doc(alias = "offset")]
 #[allow(non_camel_case_types)]
 #[repr(u64)]
-#[derive(Debug)]
+#[derive(Debug, qemu_api_macros::TryInto)]
 pub enum RegisterOffset {
     /// Data Register
     ///
@@ -102,32 +102,6 @@ pub enum RegisterOffset {
     //Reserved = 0x04C,
 }
 
-impl core::convert::TryFrom<u64> for RegisterOffset {
-    type Error = u64;
-
-    fn try_from(value: u64) -> Result<Self, Self::Error> {
-        macro_rules! case {
-            ($($discriminant:ident),*$(,)*) => {
-                /* check that matching on all macro arguments compiles, which means we are not
-                 * missing any enum value; if the type definition ever changes this will stop
-                 * compiling.
-                 */
-                const fn _assert_exhaustive(val: RegisterOffset) {
-                    match val {
-                        $(RegisterOffset::$discriminant => (),)*
-                    }
-                }
-
-                match value {
-                    $(x if x == Self::$discriminant as u64 => Ok(Self::$discriminant),)*
-                     _ => Err(value),
-                }
-            }
-        }
-        case! { DR, RSR, FR, FBRD, ILPR, IBRD, LCR_H, CR, FLS, IMSC, RIS, MIS, ICR, DMACR }
-    }
-}
-
 pub mod registers {
     //! Device registers exposed as typed structs which are backed by arbitrary
     //! integer bitmaps. [`Data`], [`Control`], [`LineControl`], etc.
diff --git a/rust/qemu-api-macros/src/lib.rs b/rust/qemu-api-macros/src/lib.rs
index 74a8bc7..7ec2182 100644
--- a/rust/qemu-api-macros/src/lib.rs
+++ b/rust/qemu-api-macros/src/lib.rs
@@ -3,75 +3,81 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 use proc_macro::TokenStream;
-use proc_macro2::Span;
-use quote::{quote, quote_spanned};
+use quote::quote;
 use syn::{
-    parse_macro_input, parse_quote, punctuated::Punctuated, token::Comma, Data, DeriveInput, Field,
-    Fields, Ident, Type, Visibility,
+    parse_macro_input, parse_quote, punctuated::Punctuated, spanned::Spanned, token::Comma, Data,
+    DeriveInput, Field, Fields, Ident, Meta, Path, Token, Type, Variant, Visibility,
 };
 
-struct CompileError(String, Span);
+mod utils;
+use utils::MacroError;
 
-impl From<CompileError> for proc_macro2::TokenStream {
-    fn from(err: CompileError) -> Self {
-        let CompileError(msg, span) = err;
-        quote_spanned! { span => compile_error!(#msg); }
+fn get_fields<'a>(
+    input: &'a DeriveInput,
+    msg: &str,
+) -> Result<&'a Punctuated<Field, Comma>, MacroError> {
+    if let Data::Struct(s) = &input.data {
+        if let Fields::Named(fs) = &s.fields {
+            Ok(&fs.named)
+        } else {
+            Err(MacroError::Message(
+                format!("Named fields required for {}", msg),
+                input.ident.span(),
+            ))
+        }
+    } else {
+        Err(MacroError::Message(
+            format!("Struct required for {}", msg),
+            input.ident.span(),
+        ))
     }
 }
 
-fn is_c_repr(input: &DeriveInput, msg: &str) -> Result<(), CompileError> {
+fn is_c_repr(input: &DeriveInput, msg: &str) -> Result<(), MacroError> {
     let expected = parse_quote! { #[repr(C)] };
 
     if input.attrs.iter().any(|attr| attr == &expected) {
         Ok(())
     } else {
-        Err(CompileError(
+        Err(MacroError::Message(
             format!("#[repr(C)] required for {}", msg),
             input.ident.span(),
         ))
     }
 }
 
-#[proc_macro_derive(Object)]
-pub fn derive_object(input: TokenStream) -> TokenStream {
-    let input = parse_macro_input!(input as DeriveInput);
-    let name = input.ident;
+fn derive_object_or_error(input: DeriveInput) -> Result<proc_macro2::TokenStream, MacroError> {
+    is_c_repr(&input, "#[derive(Object)]")?;
+
+    let name = &input.ident;
+    let parent = &get_fields(&input, "#[derive(Object)]")?[0].ident;
+
+    Ok(quote! {
+        ::qemu_api::assert_field_type!(#name, #parent,
+            ::qemu_api::qom::ParentField<<#name as ::qemu_api::qom::ObjectImpl>::ParentType>);
 
-    let expanded = quote! {
         ::qemu_api::module_init! {
             MODULE_INIT_QOM => unsafe {
                 ::qemu_api::bindings::type_register_static(&<#name as ::qemu_api::qom::ObjectImpl>::TYPE_INFO);
             }
         }
-    };
-
-    TokenStream::from(expanded)
+    })
 }
 
-fn get_fields(input: &DeriveInput) -> Result<&Punctuated<Field, Comma>, CompileError> {
-    if let Data::Struct(s) = &input.data {
-        if let Fields::Named(fs) = &s.fields {
-            Ok(&fs.named)
-        } else {
-            Err(CompileError(
-                "Cannot generate offsets for unnamed fields.".to_string(),
-                input.ident.span(),
-            ))
-        }
-    } else {
-        Err(CompileError(
-            "Cannot generate offsets for union or enum.".to_string(),
-            input.ident.span(),
-        ))
-    }
+#[proc_macro_derive(Object)]
+pub fn derive_object(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    let expanded = derive_object_or_error(input).unwrap_or_else(Into::into);
+
+    TokenStream::from(expanded)
 }
 
 #[rustfmt::skip::macros(quote)]
-fn derive_offsets_or_error(input: DeriveInput) -> Result<proc_macro2::TokenStream, CompileError> {
+fn derive_offsets_or_error(input: DeriveInput) -> Result<proc_macro2::TokenStream, MacroError> {
     is_c_repr(&input, "#[derive(offsets)]")?;
 
     let name = &input.ident;
-    let fields = get_fields(&input)?;
+    let fields = get_fields(&input, "#[derive(offsets)]")?;
     let field_names: Vec<&Ident> = fields.iter().map(|f| f.ident.as_ref().unwrap()).collect();
     let field_types: Vec<&Type> = fields.iter().map(|f| &f.ty).collect();
     let field_vis: Vec<&Visibility> = fields.iter().map(|f| &f.vis).collect();
@@ -92,3 +98,73 @@ pub fn derive_offsets(input: TokenStream) -> TokenStream {
 
     TokenStream::from(expanded)
 }
+
+#[allow(non_snake_case)]
+fn get_repr_uN(input: &DeriveInput, msg: &str) -> Result<Path, MacroError> {
+    let repr = input.attrs.iter().find(|attr| attr.path().is_ident("repr"));
+    if let Some(repr) = repr {
+        let nested = repr.parse_args_with(Punctuated::<Meta, Token![,]>::parse_terminated)?;
+        for meta in nested {
+            match meta {
+                Meta::Path(path) if path.is_ident("u8") => return Ok(path),
+                Meta::Path(path) if path.is_ident("u16") => return Ok(path),
+                Meta::Path(path) if path.is_ident("u32") => return Ok(path),
+                Meta::Path(path) if path.is_ident("u64") => return Ok(path),
+                _ => {}
+            }
+        }
+    }
+
+    Err(MacroError::Message(
+        format!("#[repr(u8/u16/u32/u64) required for {}", msg),
+        input.ident.span(),
+    ))
+}
+
+fn get_variants(input: &DeriveInput) -> Result<&Punctuated<Variant, Comma>, MacroError> {
+    if let Data::Enum(e) = &input.data {
+        if let Some(v) = e.variants.iter().find(|v| v.fields != Fields::Unit) {
+            return Err(MacroError::Message(
+                "Cannot derive TryInto for enum with non-unit variants.".to_string(),
+                v.fields.span(),
+            ));
+        }
+        Ok(&e.variants)
+    } else {
+        Err(MacroError::Message(
+            "Cannot derive TryInto for union or struct.".to_string(),
+            input.ident.span(),
+        ))
+    }
+}
+
+#[rustfmt::skip::macros(quote)]
+fn derive_tryinto_or_error(input: DeriveInput) -> Result<proc_macro2::TokenStream, MacroError> {
+    let repr = get_repr_uN(&input, "#[derive(TryInto)]")?;
+
+    let name = &input.ident;
+    let variants = get_variants(&input)?;
+    let discriminants: Vec<&Ident> = variants.iter().map(|f| &f.ident).collect();
+
+    Ok(quote! {
+        impl core::convert::TryFrom<#repr> for #name {
+            type Error = #repr;
+
+            fn try_from(value: #repr) -> Result<Self, Self::Error> {
+                #(const #discriminants: #repr = #name::#discriminants as #repr;)*;
+                match value {
+                    #(#discriminants => Ok(Self::#discriminants),)*
+                    _ => Err(value),
+                }
+            }
+        }
+    })
+}
+
+#[proc_macro_derive(TryInto)]
+pub fn derive_tryinto(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    let expanded = derive_tryinto_or_error(input).unwrap_or_else(Into::into);
+
+    TokenStream::from(expanded)
+}
diff --git a/rust/qemu-api-macros/src/utils.rs b/rust/qemu-api-macros/src/utils.rs
new file mode 100644
index 0000000..02c91ae
--- /dev/null
+++ b/rust/qemu-api-macros/src/utils.rs
@@ -0,0 +1,26 @@
+// Procedural macro utilities.
+// Author(s): Paolo Bonzini <pbonzini@redhat.com>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+use proc_macro2::Span;
+use quote::quote_spanned;
+
+pub enum MacroError {
+    Message(String, Span),
+    ParseError(syn::Error),
+}
+
+impl From<syn::Error> for MacroError {
+    fn from(err: syn::Error) -> Self {
+        MacroError::ParseError(err)
+    }
+}
+
+impl From<MacroError> for proc_macro2::TokenStream {
+    fn from(err: MacroError) -> Self {
+        match err {
+            MacroError::Message(msg, span) => quote_spanned! { span => compile_error!(#msg); },
+            MacroError::ParseError(err) => err.into_compile_error(),
+        }
+    }
+}
diff --git a/rust/qemu-api/meson.build b/rust/qemu-api/meson.build
index ccb20f3..60944a6 100644
--- a/rust/qemu-api/meson.build
+++ b/rust/qemu-api/meson.build
@@ -7,7 +7,7 @@ if rustc.version().version_compare('>=1.77.0')
   _qemu_api_cfg += ['--cfg', 'has_offset_of']
 endif
 if get_option('debug_mutex')
-  _qemu_api_cfg += ['--feature', 'debug_cell']
+  _qemu_api_cfg += ['--cfg', 'feature="debug_cell"']
 endif
 
 _qemu_api_rs = static_library(
@@ -15,6 +15,7 @@ _qemu_api_rs = static_library(
   structured_sources(
     [
       'src/lib.rs',
+      'src/assertions.rs',
       'src/bindings.rs',
       'src/bitops.rs',
       'src/callbacks.rs',
diff --git a/rust/qemu-api/src/assertions.rs b/rust/qemu-api/src/assertions.rs
new file mode 100644
index 0000000..6e42046
--- /dev/null
+++ b/rust/qemu-api/src/assertions.rs
@@ -0,0 +1,90 @@
+// Copyright 2024, Red Hat Inc.
+// Author(s): Paolo Bonzini <pbonzini@redhat.com>
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! This module provides macros to check the equality of types and
+//! the type of `struct` fields.  This can be useful to ensure that
+//! types match the expectations of C code.
+
+// Based on https://stackoverflow.com/questions/64251852/x/70978292#70978292
+// (stackoverflow answers are released under MIT license).
+
+#[doc(hidden)]
+pub trait EqType {
+    type Itself;
+}
+
+impl<T> EqType for T {
+    type Itself = T;
+}
+
+/// Assert that two types are the same.
+///
+/// # Examples
+///
+/// ```
+/// # use qemu_api::assert_same_type;
+/// # use std::ops::Deref;
+/// assert_same_type!(u32, u32);
+/// assert_same_type!(<Box<u32> as Deref>::Target, u32);
+/// ```
+///
+/// Different types will cause a compile failure
+///
+/// ```compile_fail
+/// # use qemu_api::assert_same_type;
+/// assert_same_type!(&Box<u32>, &u32);
+/// ```
+#[macro_export]
+macro_rules! assert_same_type {
+    ($t1:ty, $t2:ty) => {
+        const _: () = {
+            #[allow(unused)]
+            fn assert_same_type(v: $t1) {
+                fn types_must_be_equal<T, U>(_: T)
+                where
+                    T: $crate::assertions::EqType<Itself = U>,
+                {
+                }
+                types_must_be_equal::<_, $t2>(v);
+            }
+        };
+    };
+}
+
+/// Assert that a field of a struct has the given type.
+///
+/// # Examples
+///
+/// ```
+/// # use qemu_api::assert_field_type;
+/// pub struct A {
+///     field1: u32,
+/// }
+///
+/// assert_field_type!(A, field1, u32);
+/// ```
+///
+/// Different types will cause a compile failure
+///
+/// ```compile_fail
+/// # use qemu_api::assert_field_type;
+/// # pub struct A { field1: u32 }
+/// assert_field_type!(A, field1, i32);
+/// ```
+#[macro_export]
+macro_rules! assert_field_type {
+    ($t:ty, $i:tt, $ti:ty) => {
+        const _: () = {
+            #[allow(unused)]
+            fn assert_field_type(v: $t) {
+                fn types_must_be_equal<T, U>(_: T)
+                where
+                    T: $crate::assertions::EqType<Itself = U>,
+                {
+                }
+                types_must_be_equal::<_, $ti>(v.$i);
+            }
+        };
+    };
+}
diff --git a/rust/qemu-api/src/irq.rs b/rust/qemu-api/src/irq.rs
index 6258141..378e520 100644
--- a/rust/qemu-api/src/irq.rs
+++ b/rust/qemu-api/src/irq.rs
@@ -24,8 +24,7 @@ use crate::{
 ///
 /// Interrupts are implemented as a pointer to the interrupt "sink", which has
 /// type [`IRQState`].  A device exposes its source as a QOM link property using
-/// a function such as
-/// [`SysBusDevice::init_irq`](crate::sysbus::SysBusDevice::init_irq), and
+/// a function such as [`SysBusDeviceMethods::init_irq`], and
 /// initially leaves the pointer to a NULL value, representing an unconnected
 /// interrupt. To connect it, whoever creates the device fills the pointer with
 /// the sink's `IRQState *`, for example using `sysbus_connect_irq`.  Because
diff --git a/rust/qemu-api/src/lib.rs b/rust/qemu-api/src/lib.rs
index 4b43e02..83c6a98 100644
--- a/rust/qemu-api/src/lib.rs
+++ b/rust/qemu-api/src/lib.rs
@@ -12,6 +12,7 @@ pub mod bindings;
 #[rustfmt::skip]
 pub mod prelude;
 
+pub mod assertions;
 pub mod bitops;
 pub mod c_str;
 pub mod callbacks;
diff --git a/rust/qemu-api/src/prelude.rs b/rust/qemu-api/src/prelude.rs
index 6f32dee..4ea70b9 100644
--- a/rust/qemu-api/src/prelude.rs
+++ b/rust/qemu-api/src/prelude.rs
@@ -16,3 +16,5 @@ pub use crate::qom::ObjectMethods;
 pub use crate::qom::ObjectType;
 
 pub use crate::qom_isa;
+
+pub use crate::sysbus::SysBusDeviceMethods;
diff --git a/rust/qemu-api/src/qom.rs b/rust/qemu-api/src/qom.rs
index 7d5fbef..97901fb 100644
--- a/rust/qemu-api/src/qom.rs
+++ b/rust/qemu-api/src/qom.rs
@@ -55,6 +55,7 @@
 
 use std::{
     ffi::CStr,
+    fmt,
     ops::{Deref, DerefMut},
     os::raw::c_void,
 };
@@ -105,6 +106,52 @@ macro_rules! qom_isa {
     };
 }
 
+/// This is the same as [`ManuallyDrop<T>`](std::mem::ManuallyDrop), though
+/// it hides the standard methods of `ManuallyDrop`.
+///
+/// The first field of an `ObjectType` must be of type `ParentField<T>`.
+/// (Technically, this is only necessary if there is at least one Rust
+/// superclass in the hierarchy).  This is to ensure that the parent field is
+/// dropped after the subclass; this drop order is enforced by the C
+/// `object_deinit` function.
+///
+/// # Examples
+///
+/// ```ignore
+/// #[repr(C)]
+/// #[derive(qemu_api_macros::Object)]
+/// pub struct MyDevice {
+///     parent: ParentField<DeviceState>,
+///     ...
+/// }
+/// ```
+#[derive(Debug)]
+#[repr(transparent)]
+pub struct ParentField<T: ObjectType>(std::mem::ManuallyDrop<T>);
+
+impl<T: ObjectType> Deref for ParentField<T> {
+    type Target = T;
+
+    #[inline(always)]
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<T: ObjectType> DerefMut for ParentField<T> {
+    #[inline(always)]
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+impl<T: fmt::Display + ObjectType> fmt::Display for ParentField<T> {
+    #[inline(always)]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
+        self.0.fmt(f)
+    }
+}
+
 unsafe extern "C" fn rust_instance_init<T: ObjectImpl>(obj: *mut Object) {
     // SAFETY: obj is an instance of T, since rust_instance_init<T>
     // is called from QOM core as the instance_init function
@@ -116,11 +163,7 @@ unsafe extern "C" fn rust_instance_post_init<T: ObjectImpl>(obj: *mut Object) {
     // SAFETY: obj is an instance of T, since rust_instance_post_init<T>
     // is called from QOM core as the instance_post_init function
     // for class T
-    //
-    // FIXME: it's not really guaranteed that there are no backpointers to
-    // obj; it's quite possible that they have been created by instance_init().
-    // The receiver should be &self, not &mut self.
-    T::INSTANCE_POST_INIT.unwrap()(unsafe { &mut *obj.cast::<T>() })
+    T::INSTANCE_POST_INIT.unwrap()(unsafe { &*obj.cast::<T>() })
 }
 
 unsafe extern "C" fn rust_class_init<T: ObjectType + ClassInitImpl<T::Class>>(
@@ -133,6 +176,16 @@ unsafe extern "C" fn rust_class_init<T: ObjectType + ClassInitImpl<T::Class>>(
     T::class_init(unsafe { &mut *klass.cast::<T::Class>() })
 }
 
+unsafe extern "C" fn drop_object<T: ObjectImpl>(obj: *mut Object) {
+    // SAFETY: obj is an instance of T, since drop_object<T> is called
+    // from the QOM core function object_deinit() as the instance_finalize
+    // function for class T.  Note that while object_deinit() will drop the
+    // superclass field separately after this function returns, `T` must
+    // implement the unsafe trait ObjectType; the safety rules for the
+    // trait mandate that the parent field is manually dropped.
+    unsafe { std::ptr::drop_in_place(obj.cast::<T>()) }
+}
+
 /// Trait exposed by all structs corresponding to QOM objects.
 ///
 /// # Safety
@@ -151,11 +204,16 @@ unsafe extern "C" fn rust_class_init<T: ObjectType + ClassInitImpl<T::Class>>(
 ///
 /// - the struct must be `#[repr(C)]`;
 ///
-/// - the first field of the struct must be of the instance struct corresponding
-///   to the superclass, which is `ObjectImpl::ParentType`
+/// - the first field of the struct must be of type
+///   [`ParentField<T>`](ParentField), where `T` is the parent type
+///   [`ObjectImpl::ParentType`]
+///
+/// - the first field of the `Class` must be of the class struct corresponding
+///   to the superclass, which is `ObjectImpl::ParentType::Class`. `ParentField`
+///   is not needed here.
 ///
-/// - likewise, the first field of the `Class` must be of the class struct
-///   corresponding to the superclass, which is `ObjectImpl::ParentType::Class`.
+/// In both cases, having a separate class type is not necessary if the subclass
+/// does not add any field.
 pub unsafe trait ObjectType: Sized {
     /// The QOM class object corresponding to this struct.  This is used
     /// to automatically generate a `class_init` method.
@@ -384,13 +442,12 @@ impl<T: ObjectType> ObjectCastMut for &mut T {}
 
 /// Trait a type must implement to be registered with QEMU.
 pub trait ObjectImpl: ObjectType + ClassInitImpl<Self::Class> {
-    /// The parent of the type.  This should match the first field of
-    /// the struct that implements `ObjectImpl`:
+    /// The parent of the type.  This should match the first field of the
+    /// struct that implements `ObjectImpl`, minus the `ParentField<_>` wrapper.
     type ParentType: ObjectType;
 
     /// Whether the object can be instantiated
     const ABSTRACT: bool = false;
-    const INSTANCE_FINALIZE: Option<unsafe extern "C" fn(obj: *mut Object)> = None;
 
     /// Function that is called to initialize an object.  The parent class will
     /// have already been initialized so the type is only responsible for
@@ -402,7 +459,7 @@ pub trait ObjectImpl: ObjectType + ClassInitImpl<Self::Class> {
 
     /// Function that is called to finish initialization of an object, once
     /// `INSTANCE_INIT` functions have been called.
-    const INSTANCE_POST_INIT: Option<fn(&mut Self)> = None;
+    const INSTANCE_POST_INIT: Option<fn(&Self)> = None;
 
     /// Called on descendent classes after all parent class initialization
     /// has occurred, but before the class itself is initialized.  This
@@ -426,7 +483,7 @@ pub trait ObjectImpl: ObjectType + ClassInitImpl<Self::Class> {
             None => None,
             Some(_) => Some(rust_instance_post_init::<Self>),
         },
-        instance_finalize: Self::INSTANCE_FINALIZE,
+        instance_finalize: Some(drop_object::<Self>),
         abstract_: Self::ABSTRACT,
         class_size: core::mem::size_of::<Self::Class>(),
         class_init: Some(rust_class_init::<Self>),
diff --git a/rust/qemu-api/src/sysbus.rs b/rust/qemu-api/src/sysbus.rs
index 8193734..e6762b5 100644
--- a/rust/qemu-api/src/sysbus.rs
+++ b/rust/qemu-api/src/sysbus.rs
@@ -32,20 +32,33 @@ where
     }
 }
 
-impl SysBusDevice {
-    /// Return `self` cast to a mutable pointer, for use in calls to C code.
-    const fn as_mut_ptr(&self) -> *mut SysBusDevice {
-        addr_of!(*self) as *mut _
+/// Trait for methods of [`SysBusDevice`] and its subclasses.
+pub trait SysBusDeviceMethods: ObjectDeref
+where
+    Self::Target: IsA<SysBusDevice>,
+{
+    /// Expose a memory region to the board so that it can give it an address
+    /// in guest memory.  Note that the ordering of calls to `init_mmio` is
+    /// important, since whoever creates the sysbus device will refer to the
+    /// region with a number that corresponds to the order of calls to
+    /// `init_mmio`.
+    fn init_mmio(&self, iomem: &bindings::MemoryRegion) {
+        assert!(bql_locked());
+        unsafe {
+            bindings::sysbus_init_mmio(self.as_mut_ptr(), addr_of!(*iomem) as *mut _);
+        }
     }
 
     /// Expose an interrupt source outside the device as a qdev GPIO output.
     /// Note that the ordering of calls to `init_irq` is important, since
     /// whoever creates the sysbus device will refer to the interrupts with
     /// a number that corresponds to the order of calls to `init_irq`.
-    pub fn init_irq(&self, irq: &InterruptSource) {
+    fn init_irq(&self, irq: &InterruptSource) {
         assert!(bql_locked());
         unsafe {
             bindings::sysbus_init_irq(self.as_mut_ptr(), irq.as_ptr());
         }
     }
 }
+
+impl<R: ObjectDeref> SysBusDeviceMethods for R where R::Target: IsA<SysBusDevice> {}
diff --git a/rust/qemu-api/tests/tests.rs b/rust/qemu-api/tests/tests.rs
index 1d2825b..526c3f4 100644
--- a/rust/qemu-api/tests/tests.rs
+++ b/rust/qemu-api/tests/tests.rs
@@ -15,7 +15,7 @@ use qemu_api::{
     declare_properties, define_property,
     prelude::*,
     qdev::{DeviceImpl, DeviceState, Property},
-    qom::ObjectImpl,
+    qom::{ObjectImpl, ParentField},
     vmstate::VMStateDescription,
     zeroable::Zeroable,
 };
@@ -31,7 +31,7 @@ pub static VMSTATE: VMStateDescription = VMStateDescription {
 #[repr(C)]
 #[derive(qemu_api_macros::Object)]
 pub struct DummyState {
-    parent: DeviceState,
+    parent: ParentField<DeviceState>,
     migrate_clock: bool,
 }