💾 Archived View for gmi.noulin.net › man › man2 › subpage_prot.2.gmi captured on 2022-06-12 at 06:41:11. Gemini links have been rewritten to link to archived content

View Raw

More Information

-=-=-=-=-=-=-

SUBPAGE_PROT(2)                                                         Linux Programmer's Manual                                                        SUBPAGE_PROT(2)

NAME
       subpage_prot - define a subpage protection for an address range

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 re‐
              gion 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  CON‐
       FIG_PPC_64K_PAGES.  No library support is provided.

CONFORMING TO
       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

Linux                                                                          2021-03-22                                                                SUBPAGE_PROT(2)