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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, 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 128K
Rules 1) and 2) apply to slab_alloc as well. Using SLAB allocator
to allocate too large values is not recommended.