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.

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.

STANDARDS

       This system call is Linux-specific.

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