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-.\" Copyright (c) 2010 Michael Kerrisk <mtk.manpages@gmail.com>
-.\" based on a proposal from Stephan Mueller <smueller@atsec.com>
-.\"
-.\" SPDX-License-Identifier: Linux-man-pages-copyleft
-.\"
-.\" Various pieces of text taken from the kernel source and the commentary
-.\" in Linux commit fa28237cfcc5827553044cbd6ee52e33692b0faa
-.\" both written by Paul Mackerras <paulus@samba.org>
-.\"
-.TH subpage_prot 2 (date) "Linux man-pages (unreleased)"
-.SH NAME
-subpage_prot \- define a subpage protection for an address range
-.SH LIBRARY
-Standard C library
-.RI ( libc ", " \-lc )
-.SH SYNOPSIS
-.nf
-.BR "#include <sys/syscall.h>" "      /* Definition of " SYS_* " constants */"
-.B #include <unistd.h>
-.P
-.BI "int syscall(SYS_subpage_prot, unsigned long " addr ", unsigned long " len ,
-.BI "            uint32_t *" map );
-.fi
-.P
-.IR Note :
-glibc provides no wrapper for
-.BR subpage_prot (),
-necessitating the use of
-.BR syscall (2).
-.SH DESCRIPTION
-The PowerPC-specific
-.BR 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.
-.P
-The protection map is applied to the memory pages in the region starting at
-.I addr
-and continuing for
-.I len
-bytes.
-Both of these arguments must be aligned to a 64-kB boundary.
-.P
-The protection map is specified in the buffer pointed to by
-.IR 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
-.I map
-should equate to the number of 64-kB pages specified by
-.IR 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.
-.SH RETURN VALUE
-On success,
-.BR subpage_prot ()
-returns 0.
-Otherwise, one of the error codes specified below is returned.
-.SH ERRORS
-.TP
-.B EFAULT
-The buffer referred to by
-.I map
-is not accessible.
-.TP
-.B EINVAL
-The
-.I addr
-or
-.I 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.
-.TP
-.B ENOMEM
-Out of memory.
-.SH STANDARDS
-Linux.
-.SH HISTORY
-Linux 2.6.25 (PowerPC).
-.P
-The system call is provided only if the kernel is configured with
-.BR CONFIG_PPC_64K_PAGES .
-.SH 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.
-.SS 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).
-.\" In the initial implementation, it was the case that:
-.\"     In fact the whole process is switched to use 4 kB hardware pages when the
-.\"     subpage_prot system call is used, but this could be improved in future
-.\"     to switch only the affected segments.
-.\" But Paul Mackerass says (Oct 2010): I'm pretty sure we now only switch
-.\" the affected segment, not the whole process.
-.SH SEE ALSO
-.BR mprotect (2),
-.BR syscall (2)
-.P
-.I Documentation/admin\-guide/mm/hugetlbpage.rst
-in the Linux kernel source tree