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

Linux man-pages 6.7                                2023-10-31                                    subpage_prot(2)