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

NAME

     subpage_prot - define a subpage protection for an address range

SYNOPSIS

     long 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  sys-
     tems configured with a page size of 64 kB.

     The  protection map is applied to the memory pages in the region start-
     ing 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  argu-
            ments 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  sub-
     page  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-pro-
     tected.

 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 pro-
     tection 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).