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Memory management
=================

1. Virtual Address Translation

1.1 Hierarchical 4-level per address space page tables

SPARTAN kernel deploys generic interface for 4-level page tables for these
architectures: amd64, arm32, ia32, mips32 and ppc32. In this setting, page
tables are hierarchical and are not shared by address spaces (i.e. one set of
page tables per address space).


 VADDR
 +-----------------------------------------------------------------------------+
 |   PTL0_INDEX  |   PTL1_INDEX   |   PTL2_INDEX   |   PTL3_INDEX   |   OFFSET |
 +-----------------------------------------------------------------------------+


 PTL0                   PTL1                   PTL2                   PTL3
 +--------+             +--------+             +--------+             +--------+
 |        |             |        |             |  PTL3  | -----\      |        |
 |        |             |        |             +--------+      |      |        |
 |        |             +--------+             |        |      |      |        |
 |        |             |  PTL2  | -----\      |        |      |      |        |
 |        |             +--------+      |      |        |      |      |        |
 |        |             |        |      |      |        |      |      +--------+
 +--------+             |        |      |      |        |      |      | FRAME  |
 |  PTL1  | -----\      |        |      |      |        |      |      +--------+
 +--------+      |      |        |      |      |        |      |      |        |
 |        |      |      |        |      |      |        |      |      |        |
 |        |      |      |        |      |      |        |      |      |        |
 +--------+      \----> +--------+      \----> +--------+      \----> +--------+
     ^
     |
     |
 +--------+
 |  PTL0  |
 +--------+


PTL0        Page Table Level 0 (Page Directory)
PTL1        Page Table Level 1
PTL2        Page Table Level 2
PTL3        Page Table Level 3

PTL0_INDEX  Index into PTL0
PTL1_INDEX  Index into PTL1
PTL2_INDEX  Index into PTL2
PTL3_INDEX  Index into PTL3

VADDR       Virtual address for which mapping is looked up
FRAME       Physical address of memory frame to which VADDR is mapped


On architectures whose hardware has fewer levels, PTL2 and, if need be, PTL1 are
left out. TLB-only architectures are to define custom format for software page
tables.

1.2 Single global page hash table

Generic page hash table interface is deployed on 64-bit architectures without
implied hardware support for hierarchical page tables, i.e. ia64 and sparc64.
There is only one global page hash table in the system shared by all address
spaces.


2. Memory allocators

2.1 General allocator

'malloc' function accepts flags as a second argument. The flags are directly
passed to the underlying frame_alloc function. 

1) If the flags parameter contains FRAME_ATOMIC, the allocator will not sleep.
   The allocator CAN return NULL, when memory is not directly available.
   The caller MUST check if NULL was not returned

2) If the flags parameter does not contain FRAME_ATOMIC, the allocator
   will never return NULL, but it CAN sleep indefinitely. The caller
   does not have to check the return value.

3) The maximum size that can be allocated using malloc is 256K

Rules 1) and 2) apply to slab_alloc as well. Using SLAB allocator
to allocate too large values is not recommended.