49. Virtual mapping¶
49.1. Introduction¶
.intro: This the design of the VM interface. The VM interface
provides a simple, low-level, operating-system independent interface
to address-space. Each call to VMInit() reserves (from the
operating-system) a single contiguous range of addresses, and updates
a VMStruct thereafter used to manage this address-space. The VM
interface has separate implementations for each platform that supports
it (at least conceptually, in practice some of them may be the same).
The VM module provides a mechanism to reserve large (relative to the
amount of RAM) amounts of address space, and functions to map (back
with RAM) and unmap portions of this address space.
.motivation: The VM is used by the VM Arena Class. It provides the basic substrate to provide sparse address maps. Sparse address maps have at least two uses: to encode information into the address of an object which is used in tracing (the Zone Test) to speed things up; to avoid fragmentation at the segment level and above (since the amount of address space reserved is large compared to the RAM, the hope is that there will also be enough address space somewhere to fit any particular segment in).
49.2. Definitions¶
.def.reserve: The reserve operation: exclusively reserve a portion of the virtual address space without arranging RAM or backing store for the virtual addresses. The intention is that no other component in the process will make use of the reserved virtual addresses, but in practice this may entail assuming a certain amount of cooperation. When reserving address space, the requester simply asks for a particular size, not a particular range of virtual addresses. Accessing (read/write/execute) reserved addresses is illegal unless those addresses have been mapped.
.def.map: The map operation: arrange that a specified portion of the virtual address space is mapped from the swap, effectively allocating RAM and/or swap space for a particular range of addresses. If successful, accessing the addresses is now legal. Only reserved addresses should be mapped.
.def.unmap: The unmap operation: the inverse of the map operation. Arrange that a specified portion of the virtual address space is no longer mapped, effectively freeing up the RAM and swap space that was in use. Accessing the addresses is now illegal. The addresses return to the reserved state.
.def.vm: “VM” stands for Virtual Memory. Various meanings: a
processor architecture’s virtual space and structure; the generic
idea, interface, or implementation of the MPS VM module; the C
structure (struct VMStruct) used to encapsulate the functionality
of the MPS VM module; an instance of such a structure.
.def.vm.mps: In the MPS, a “VM” is a VMStruct, providing access
to the single contiguous range of address-space that was reserved
(from the operating-system) when VMInit() was called.
49.3. Interface¶
.if.page.size: Return the operating system’s “page size”, that is,
the granularity with which the operating system can map and unmap
virtual memory. For speed (on systems like Windows where determining
the page size involves a system call), this is cached in each VM
descriptor and accessible via the VMPageSize() function.
.if.param.from.args: This function processes the keyword arguments
(the ones that are relevant to the VM implementation) in the args
parameter and stores a description of them in the buffer pointed to by
params (which is paramSize bytes long). It is an error if the
buffer is not big enough store the parameters for this VM
implementation.
.if.init: VMInit() is responsible for reserving an amount
of virtual address space. The params argument points to a
parameter block initialized by a call to VMParamFromArgs(). If
successful, the VM descriptor given by the parameter vm is
updated to describe the address space, and ResOK is returned. The
created VM has at least size bytes of virtual memory reserved
starting at an address which is a multiple of grainSize.
If there’s not enough address space to reserve a block of the given
size, ResRESOURCE is returned. The reserved virtual memory can be
mapped and unmapped by calling VMMap() and VMUnmap().
-
void
VMFinish(VM vm)¶
.if.finish: A VM is destroyed by calling VMFinish(). Any
address space that was mapped through this VM is unmapped.
.if.map: Map the range of addresses from base (inclusive) to
limit (exclusive) into memory. It is an error if the range does
not lie between VMBase(vm) and VMLimit(vm), or if base and
limit are not multiples of VMPageSize(vm). Return ResOK
if successful, ResMEMORY if not.
.if.unmap: Unmap the range of addresses from base (inclusive)
to limit (exclusive). The conditions are the same as for
VMMap().
.if.base: Return the base address of the VM (the lowest address in the VM that is a multiple of the grain size).
.if.limit: Return the limit address of the VM (the limit of the last grain that is wholly inside the VM).
.if.reserved: Return the total number of bytes of address space
reserved by the VM. (This may include addresses that are not available
for mapping because of the requirement for VMBase(vm) and
VMLimit(vm) to be multiples of the grain size.)
.if.mapped: Return the total number of bytes of address space currently mapped into memory by the VM.
49.4. Notes¶
.testing: It is important to test that a VM implementation will work in extreme cases.
.testing.large: It must be able to reserve a large address space.
Clients will want multi-GB spaces, more than that OSs will allow. If
they ask for too much, mps_arena_create() (and hence
VMInit()) must fail in a predictable way.
.testing.larger: It must be possible to allocate in a large space; sometimes commiting will fail, because there’s not enough space to replace the “reserve” mapping. See request.epcore.160201 for details.
.testing.lots: It must be possible to have lots of mappings. The OS must either combine adjacent mappings or have lots of space in the kernel tables. See request.epcore.160117 for ideas on how to test this.
