subpage_prot - define a subpage protection for an address range
long subpage_prot(unsigned long addr, unsigned long len,
: There is no glibc wrapper for this system call; see NOTES.
The PowerPC-specific subpage_prot
() system call provides the facility to
control the access permissions on individual 4 kB subpages on systems
configured with a page size of 64 kB.
The protection map is applied to the memory pages in the region starting at
and continuing for len
bytes. Both of these arguments must
be aligned to a 64-kB boundary.
The protection map is specified in the buffer pointed to by map
. The map
has 2 bits per 4 kB subpage; thus each 32-bit word specifies the
protections of 16 4 kB subpages inside a 64 kB page (so, the
number of 32-bit words pointed to by map
should equate to the number of
64-kB pages specified by len
). Each 2-bit field in the protection map
is either 0 to allow any access, 1 to prevent writes, or 2 or 3 to prevent all
On success, subpage_prot
() returns 0. Otherwise, one of the error codes
specified below is returned.
- The buffer referred to by map is not accessible.
- The addr or len arguments are incorrect. Both of these
arguments must be aligned to a multiple of the system page size, and they
must not refer to a region outside of the address space of the process or
to a region that consists of huge pages.
- Out of memory.
This system call is provided on the PowerPC architecture since Linux 2.6.25. The
system call is provided only if the kernel is configured with
. No library support is provided.
This system call is Linux-specific.
Glibc does not provide a wrapper for this system call; call it using
Normal page protections (at the 64-kB page level) also apply; the subpage
protection mechanism is an additional constraint, so putting 0 in a 2-bit
field won't allow writes to a page that is otherwise write-protected.
This system call is provided to assist writing emulators that operate using
64-kB pages on PowerPC systems. When emulating systems such as x86, which uses
a smaller page size, the emulator can no longer use the memory-management unit
(MMU) and normal system calls for controlling page protections. (The emulator
could emulate the MMU by checking and possibly remapping the address for each
memory access in software, but that is slow.) The idea is that the emulator
supplies an array of protection masks to apply to a specified range of virtual
addresses. These masks are applied at the level where hardware page-table
entries (PTEs) are inserted into the hardware page table based on the Linux
PTEs, so the Linux PTEs are not affected. Implicit in this is that the regions
of the address space that are protected are switched to use 4-kB hardware
pages rather than 64-kB hardware pages (on machines with hardware 64-kB page
in the Linux kernel source