SUBPAGE_PROT
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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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 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/vm/hugetlbpage.txt
in the Linux kernel source tree
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- Rationale
-
- SEE ALSO
-
- COLOPHON
-
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