SUBPAGE_PROT(2)           (2020-12-21)            SUBPAGE_PROT(2)

     NAME
          subpage_prot - define a subpage protection for an address
          range

     SYNOPSIS
          int 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
          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

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     SUBPAGE_PROT(2)           (2020-12-21)            SUBPAGE_PROT(2)

          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 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 pro-
          tections.  (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 speci-
          fied 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 sup-
          port).

     SEE ALSO
          mprotect(2), syscall(2)

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

     COLOPHON
          This page is part of release 5.10 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/.

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