This vmm package exports ReservedZeroedFrame which can be used to setup
a lazy physical page allocation scheme. This is implemented by mapping
ReservedZeroedFrame to each page in a virtual memory region using the
following flag combination: FlagPresent | FlagCopyOnWrite.
This has the effect that all reads from the virtual address region
target the contents of ReservedZeroedFrame (always returning zero). On
the other hand, writes to the virtual address region trigger a page
fault which is resolved as follows:
- a new physical frame is allocated and the contents of ReservedZeroedFrame
are copied to it (effectively clearing the new frame).
- the page entry for the virtual address that caused the fault is
updated to point to the new frame and its flags are changed to:
FlagPresent | FlagRW
- execution control is returned back to the code that caused the fault
This allows us to remove the allocFn argument from the vmm functions
which causes the compiler's escape analysis to sometimes incorectly flag
it as escaping to the heap.
The API provides the Map() and MapTemporary() functions that establish
virtual -> physical address mappings using the currently active page
directory table.
Mapped pages can be unmapped using the Unmap() function. When unmapping
virtual addresses, the page tables leading to them will not be
automatically released even if they are empty. This will be addressed by
a future commit.
