NAME

       subpage_prot - define a subpage protection for an address range

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/syscall.h>      /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_subpage_prot, unsigned long addr, unsigned long len,
                   uint32_t *map);

       Note: glibc provides no wrapper for subpage_prot(), necessitating the use of syscall(2).

DESCRIPTION

       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 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 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 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 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.

       ENOMEM Out of memory.

STANDARDS

       Linux.

HISTORY

       Linux 2.6.25 (PowerPC).

       The system call is provided only if the kernel is configured with CONFIG_PPC_64K_PAGES.

NOTES

       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.

   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 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 support).

SEE ALSO

       mprotect(2), syscall(2)

       Documentation/admin-guide/mm/hugetlbpage.rst in the Linux kernel source tree