• Jeffrey Lee's avatar
    Allow reservation of memory pages · 1f84ad9f
    Jeffrey Lee authored
    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" R0 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
    * 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)
    Version 6.28. Tagged as 'Kernel-6_28'
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