linux/arch/x86/include/asm/vvar.h

/*
 * vvar.h: Shared vDSO/kernel variable declarations
 * Copyright (c) 2011 Andy Lutomirski
 * Subject to the GNU General Public License, version 2
 *
 * A handful of variables are accessible (read-only) from userspace
 * code in the vsyscall page and the vdso.  They are declared here.
 * Some other file must define them with DEFINE_VVAR.
 *
 * In normal kernel code, they are used like any other variable.
 * In user code, they are accessed through the VVAR macro.
 *
 * These variables live in a page of kernel data that has an extra RO
 * mapping for userspace.  Each variable needs a unique offset within
 * that page; specify that offset with the DECLARE_VVAR macro.  (If
 * you mess up, the linker will catch it.)
 */

#ifndef _ASM_X86_VVAR_H
#define _ASM_X86_VVAR_H

#if defined(__VVAR_KERNEL_LDS)

/* The kernel linker script defines its own magic to put vvars in the
 * right place.
 */
#define DECLARE_VVAR(offset, type, name) \
	EMIT_VVAR(name, offset)

#else

extern char __vvar_page;

#define DECLARE_VVAR(offset, type, name)				\
	extern type vvar_ ## name __attribute__((visibility("hidden")));

#define VVAR(name) (vvar_ ## name)

#define DEFINE_VVAR(type, name)						\
	type name							\
	__attribute__((section(".vvar_" #name), aligned(16))) __visible

#endif

/* DECLARE_VVAR(offset, type, name) */

DECLARE_VVAR(128, struct vsyscall_gtod_data, vsyscall_gtod_data)

#undef DECLARE_VVAR

#endif