Allow reservation of memory pages
This change adds a new OS_Memory reason code, 23, for reserving memory without actually assigning it to a dynamic area. Other dynamic areas can still use the memory, but only the code that reserved it will be allowed to claim exclusive use over it (i.e. PageFlags_Unavailable).
This is useful for systems such as the PCI heap, where physically contiguous memory is required, but the memory isn't needed all of the time. By reserving the pages, it allows other regular DAs to make use of the memory when the PCI heap is small. But when the PCI heap needs to grow, it guarantees that (if there's enough free memory in the system) the previously reserved pages can be allocated to the PCI heap.
- Reservations are handled on an honour system; there's no checking that the program that reserved the memory is the one attempting to map it in.
- For regular NeedsSpecificPages DAs, reserved pages can only be used if the special "RESV" return value is used.
- For PMP DAs, reserved pages can only be made Unavailable if the entry in the page block also specifies the Reserved page flag. The actual state of the Reserved flag can't be modified via PMP DA ops, the flag is only used to indicate the caller's permission/intent to make the page Unavailable.
- If a PMP DA tries to make a Reserved page Unavailable without specifying the Reserved flag, the kernel will try to swap it out for a replacement page taken from the free pool (preserving the contents and generating Service_PagesUnsafe / Service_PagesSafe, as if another DA had claimed the page)