THE DESIGN OF THE VIRTUAL MAPPING INTERFACE
                             design.mps.vm
                           incomplete design
                           richard 1998-05-11

.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 VMCreate() reserves (from the operating-system) a single 
contiguous range of addresses, and returns 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).


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 / 
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 VMCreate was called.


INTERFACE

.if.create: Res VMCreate(VM *VMReturn, Size size)

VMCreate is responsible both for allocating a VMStruct and for reserving an 
amount of virtual address space.  A VM is created and a pointer to it is 
returned in the return parameter VMReturn.  This VM has at least size bytes of 
virtual memory reserved.  If there's not enough space to allocate the VM, 
ResMEMORY is returned.  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 using VMMap and VMUnmap.


.if.destroy: void VMDestroy(VM vm)

A VM is destroyed by calling VMDestroy.  Any address space that was mapped 
through this VM is unmapped.


[lots of interfaces missing here]


NOTES

.diagram:



.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 VMCreate) 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.