Allow setting object metadata for VM space objects. Expose VO bit under a feature.

Cargo.toml

@@ -183,6 +183,16 @@ bpftrace_workaround = []
 # users can disable such annotations by enabling this Cargo feature.
 no_mmap_annotation = []
 
+# Allow the binding to access Valid Object (VO) bit.
+# MMTk uses VO bit to identify a valid object. Thus VO bit is carefully managed by MMTk in cooperation with the binding.
+# So normally this feature is not needed, and its use should be discouraged.
+# However, in rare cases, a binding may want to directly access and manipulate VO bit. For example, a binding cannot cooperate
+# with MMTk to identify each object for setting the VO bit. This is usually due to the limitation of the VM.
+# In such cases, a binding may use this feature and manipulate the VO bit to their needs. The binding's manipulation on VO bit
+# may violate MMTk's semantics, and may result in undefined behaviors for VO bit related APIs. This feature should
+# only be used if you understand how VO bit works internally. Use at your own risk.
+vo_bit_access = []
+
 # Do not modify the following line - ci-common.sh matches it
 # -- Mutally exclusive features --
 # Only one feature from each group can be provided. Otherwise build will fail.

src/util/metadata/side_metadata/constants.rs

@@ -30,6 +30,7 @@ pub(crate) const GLOBAL_SIDE_METADATA_BASE_OFFSET: SideMetadataOffset =
     SideMetadataOffset::addr(GLOBAL_SIDE_METADATA_BASE_ADDRESS);
 
 /// Base address of VO bit, public to VM bindings which may need to use this.
+#[cfg(target_pointer_width = "64")]
 pub const VO_BIT_SIDE_METADATA_ADDR: Address =
     crate::util::metadata::vo_bit::VO_BIT_SIDE_METADATA_ADDR;
 

src/util/metadata/side_metadata/mod.rs

@@ -22,3 +22,9 @@ pub(crate) use helpers::*;
 #[allow(unused_imports)]
 pub(crate) use helpers_32::*;
 pub(crate) use sanity::SideMetadataSanity;
+
+/// Side metadata spec for VO bit. This is only available when the feature "vo_bit_access" is enabled.
+/// Check the comments on "vo_bit_access" in `Cargo.toml` before use. Use at your own risk.
+#[cfg(feature = "vo_bit_access")]
+pub const VO_BIT_SIDE_METADATA_SPEC: SideMetadataSpec =
+    crate::util::metadata::side_metadata::spec_defs::VO_BIT;

src/util/metadata/vo_bit/mod.rs

@@ -58,6 +58,7 @@ use crate::vm::VMBinding;
 pub(crate) const VO_BIT_SIDE_METADATA_SPEC: SideMetadataSpec =
     crate::util::metadata::side_metadata::spec_defs::VO_BIT;
 
+#[cfg(target_pointer_width = "64")]
 pub const VO_BIT_SIDE_METADATA_ADDR: Address = VO_BIT_SIDE_METADATA_SPEC.get_absolute_offset();
 
 /// Atomically set the VO bit for an object.
@@ -222,3 +223,23 @@ pub fn is_internal_ptr_from_vo_bit<VM: VMBinding>(
 pub unsafe fn is_vo_addr(addr: Address) -> bool {
     VO_BIT_SIDE_METADATA_SPEC.load::<u8>(addr) != 0
 }
+
+#[cfg(feature = "vm_space")]
+use crate::MMTK;
+
+/// Set VO bit for a VM space object.
+/// Objects in the VM space are allocated/managed by the binding. This functio provides a way for
+/// the binding to set VO bit for an object in the space.
+#[cfg(feature = "vm_space")]
+pub fn set_vo_bit_for_vm_space_object<VM: VMBinding>(
+    mmtk: &'static MMTK<VM>,
+    object: ObjectReference,
+) {
+    use crate::policy::space::Space;
+    debug_assert!(
+        mmtk.get_plan().base().vm_space.in_space(object),
+        "{} is not in VM space",
+        object
+    );
+    set_vo_bit(object);
+}