SUBPAGE_PROT(2)		   Linux Programmer's Manual	       SUBPAGE_PROT(2)



NAME
       subpage_prot - define a subpage protection for an address range

SYNOPSIS
       long subpage_prot(unsigned long addr, unsigned long len,
			 uint32_t *map);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION
       The  PowerPC-specific  subpage_prot() system call provides the facility
       to control the access permissions on individual 4kB subpages on systems
       configured with a page size of 64kB.

       The  protection map is applied to the memory pages in the region start-
       ing at addr 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 4kB subpage; thus each  32-bit  word	specifies  the
       protections  of	16  4kB subpages inside a 64kB 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 accesses.

RETURN VALUE
       On  success,  subpage_prot()  returns  0.   Otherwise, one of the error
       codes specified below is returned.

ERRORS
       EFAULT The buffer referred to by map is not accessible.

       EINVAL The addr or len arguments are incorrect.	Both  of  these	 argu-
	      ments 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.

       ENOMEM Out of memory.

VERSIONS
       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 CONFIG_PPC_64K_PAGES.  No library support is provided.

CONFORMING TO
       This system call is Linux-specific.

NOTES
       Glibc  does  not	 provide a wrapper for this system call; call it using
       syscall(2).

       Normal page protections (at the 64-kB page level) also apply; the  sub-
       page  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-pro-
       tected.

   Rationale
       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 pro-
       tection 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 support).

SEE ALSO
       mprotect(2), syscall(2)

       Documentation/vm/hugetlbpage.txt in the Linux kernel source tree

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2012-07-13		       SUBPAGE_PROT(2)