subpage_prot(2) System Calls Manual subpage_prot(2)
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 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 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 ei-
ther 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.
STANDARDS
This system call is Linux-specific.
NOTES
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).
SEE ALSO
mprotect(2), syscall(2)
Documentation/admin-guide/mm/hugetlbpage.rst in the Linux kernel source
tree
Linux man-pages 6.03 2022-12-04 subpage_prot(2)
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