Standard C library (libc, -lc)
#include <asm/guarded_storage.h> /* Definition of GS_* constants */ #include <sys/syscall.h> /* Definition of SYS_* constants */ #include <unistd.h> int syscall(SYS_s390_guarded_storage, int command, struct gs_cb *gs_cb);
Note: glibc provides no wrapper for s390_guarded_storage(), necessitating the use of syscall(2).
The s390_guarded_storage() system call enables the use of the Guarded Storage Facility (a z/Architecture-specific feature) for user-space processes.
The guarded storage facility is a hardware feature that allows marking up to 64 memory regions (as of z14) as guarded; reading a pointer with a newly introduced "Load Guarded" (LGG) or "Load Logical and Shift Guarded" (LLGFSG) instructions will cause a range check on the loaded value and invoke a (previously set up) user-space handler if one of the guarded regions is affected.
The command argument indicates which function to perform. The following commands are supported:
Enable the guarded storage facility for the calling task. The initial content of the guarded storage control block will be all zeros. After enablement, user-space code can use the "Load Guarded Storage Controls" (LGSC) instruction (or the load_gs_cb() function wrapper provided in the asm/guarded_storage.h header) to load an arbitrary control block. While a task is enabled, the kernel will save and restore the calling content of the guarded storage registers on context switch.
Disables the use of the guarded storage facility for the calling task. The kernel will cease to save and restore the content of the guarded storage registers, the task-specific content of these registers is lost.
Set a broadcast guarded storage control block to the one provided in the gs_cb argument. This is called per thread and associates a specific guarded storage control block with the calling task. This control block will be used in the broadcast command GS_BROADCAST.
Clears the broadcast guarded storage control block. The guarded storage control block will no longer have the association established by the GS_SET_BC_CB command.
Sends a broadcast to all thread siblings of the calling task. Every sibling that has established a broadcast guarded storage control block will load this control block and will be enabled for guarded storage. The broadcast guarded storage control block is consumed; a second broadcast without a refresh of the stored control block with GS_SET_BC_CB will not have any effect.
The gs_cb argument specifies the address of a guarded storage control block structure and is currently used only by the GS_SET_BC_CB command; all other aforementioned commands ignore this argument.
On success, the return value of s390_guarded_storage() is 0.
On error, -1 is returned, and errno is set to indicate the error.
command was GS_SET_BC_CB and the copying of the guarded storage control block structure pointed by the gs_cb argument has failed.
The value provided in the command argument was not valid.
command was one of GS_ENABLE or GS_SET_BC_CB, and the allocation of a new guarded storage control block has failed.
The guarded storage facility is not supported by the hardware.
Linux on s390.
Linux 4.12. System z14.
The description of the guarded storage facility along with related instructions and Guarded Storage Control Block and Guarded Storage Event Parameter List structure layouts is available in "z/Architecture Principles of Operations" beginning from the twelfth edition.
The gs_cb structure has a field gsepla (Guarded Storage Event Parameter List Address), which is a user-space pointer to a Guarded Storage Event Parameter List structure (that contains the address of the aforementioned event handler in the gseha field), and its layout is available as a gs_epl structure type definition in the asm/guarded_storage.h header.