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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
#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).