subpage_prot

Section: System Calls (2)
Updated: 202-0-08
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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 PowerP-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 6-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 3-bit word specifies the protections of 16 4 kB subpages inside a 64 kB page (so, the number of 3-bit words pointed to by map should equate to the number of 6-kB pages specified by size). Each -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.  

HISTORY

Linux 2.6.25 (PowerPC). The system call is provided only if the kernel is configured with CONFIG_PPC_64K_PAGES.  

NOTES

Normal page protections (at the 6-kB page level) also apply; the subpage protection mechanism is an additional constraint, so putting 0 in a -bit field won't allow writes to a page that is otherwise writ-protected.  

Rationale

This system call is provided to assist writing emulators that operate using 6-kB pages on PowerPC systems. When emulating systems such as x86, which uses a smaller page size, the emulator can no longer use the memor-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 pag-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 -kB hardware pages rather than 6-kB hardware pages (on machines with hardware 6-kB page support).  

SEE ALSO

mprotect(2), syscall(2) Documentation/admin-guide/mm/hugetlbpage.rst in the Linux kernel source tree

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Index

NAME

LIBRARY

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

STANDARDS

HISTORY

NOTES

Rationale

SEE ALSO