On the s390 architecture virtual kernel and user address spaces are never present at the same time, so how does copy_to_user work?
copy_to_user for s390 is implemented here: linux/arch/s390/include/asm/uaceess.h.
uaccess is the pointer to copy_[to/from]_user actual implementation. It is setted up here (grep uaccess): arch/s390/kernel/setup.c. There are 4 implementations of uaccess, depening from mode:
uaccess_mvcos_switch, uaccess_pt, uaccess_mvcos and uaccess_std
For example uaccess_std is here: http://lxr.linux.no/#linux+v3.2.1/arch/s390/lib/uaccess_std.c
4 * Standard user space access functions based on mvcp/mvcs and doing
5 * interesting things in the secondary space mode.
...
82 size_t copy_to_user_std(size_t size, void __user *ptr, const void *x)
83 {
84 unsigned long tmp1, tmp2;
85
86 tmp1 = -256UL;
87 asm volatile(
88 "0: mvcs 0(%0,%1),0(%2),%3\n"
The mvcp/mvcs mechanism is used:
http://publib.boulder.ibm.com/infocenter/zos/v1r11/topic/com.ibm.zos.r11.ieaa500/iea2a57031.htm