Struct InstanceAllocator

pub struct InstanceAllocator {
    instance_ptr: NonNull<Instance>,
    instance_layout: Layout,
    offsets: VMOffsets,
    consumed: bool,
}

This is an intermediate type that manages the raw allocation and metadata when creating a VMInstance.

This type will free the allocated memory if it’s dropped before being used.

It is important to remind that VMInstance is dynamically-sized based on VMOffsets: The Instance.vmctx field represents a dynamically-sized array that extends beyond the nominal end of the type. So in order to create an instance of it, we must:

1. Define the correct layout for Instance (size and alignment),
2. Allocate it properly.

The InstanceAllocator::instance_layout helper computes the correct layout to represent the wanted VMInstance.

Then we use this layout to allocate an empty Instance properly.

Fields

instance_ptr: NonNull<Instance>

The buffer that will contain the VMInstance and dynamic fields.

instance_layout: Layout

The layout of the instance_ptr buffer.

offsets: VMOffsets

Information about the offsets into the instance_ptr buffer for the dynamic fields.

consumed: bool

Whether or not this type has transferred ownership of the instance_ptr buffer. If it has not when being dropped, the buffer should be freed.

Implementations

impl InstanceAllocator

pub fn new( module: &ModuleInfo, ) -> (Self, Vec<NonNull<VMMemoryDefinition>>, Vec<NonNull<VMTableDefinition>>)

Allocates instance data for use with VMInstance::new.

Returns a wrapper type around the allocation and 2 vectors of pointers into the allocated buffer. These lists of pointers correspond to the location in memory for the local memories and tables respectively. These pointers should be written to before calling VMInstance::new.

fn instance_layout(offsets: &VMOffsets) -> Layout

Calculate the appropriate layout for the internal Instance structure.

unsafe fn memory_definition_locations(&self) -> Vec<NonNull<VMMemoryDefinition>>

Get the locations of where the local VMMemoryDefinitions should be stored.

This function lets us create Memory objects on the host with backing memory in the VM.

Safety

• Self.instance_ptr must point to enough memory that all of the offsets in Self.offsets point to valid locations in memory, i.e. Self.instance_ptr must have been allocated by Self::new.

unsafe fn table_definition_locations(&self) -> Vec<NonNull<VMTableDefinition>>

Get the locations of where the VMTableDefinitions should be stored.

This function lets us create [Table] objects on the host with backing memory in the VM.

Safety

• Self.instance_ptr must point to enough memory that all of the offsets in Self.offsets point to valid locations in memory, i.e. Self.instance_ptr must have been allocated by Self::new.

pub(crate) fn into_vminstance(self, instance: Instance) -> VMInstance

Finish preparing by writing the internal Instance into memory, and consume this InstanceAllocator.

pub(crate) fn offsets(&self) -> &VMOffsets

Get the VMOffsets for the allocated buffer.

Trait Implementations

impl Drop for InstanceAllocator

fn drop(&mut self)

Executes the destructor for this type.