subpage_prot

SUBPAGE_PROT(2)             Linux Programmer's Manual            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 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
       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

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/.



Linux                              2020-12-21                    SUBPAGE_PROT(2)