# subpage_prot

## NAME



           subpage_prot - define a subpage protection for an address range
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/syscall.h>      **/* Definition of **SYS_*** constants */
           **#include <unistd.h>**
    
           **int syscall(SYS_subpage_prot, unsigned long** _addr_**, unsigned long** _size_**,**
                       **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 _size_ 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 _size_).  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 _size_ 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.
    


## STANDARDS



           Linux.
    


## 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).
    


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