ARCH_PRCTL(2)               Linux Programmer's Manual              ARCH_PRCTL(2)

       arch_prctl - set architecture-specific thread state

       #include <asm/prctl.h>
       #include <sys/prctl.h>

       int arch_prctl(int code, unsigned long addr);
       int arch_prctl(int code, unsigned long *addr);

       arch_prctl() sets architecture-specific process or thread state.  code
       selects a subfunction and passes argument addr to it; addr is interpreted
       as either an unsigned long for the "set" operations, or as an unsigned
       long *, for the "get" operations.

       Subfunctions for both x86 and x86-64 are:

       ARCH_SET_CPUID (since Linux 4.12)
              Enable (addr != 0) or disable (addr == 0) the cpuid instruction
              for the calling thread.  The instruction is enabled by default.
              If disabled, any execution of a cpuid instruction will instead
              generate a SIGSEGV signal.  This feature can be used to emulate
              cpuid results that differ from what the underlying hardware would
              have produced (e.g., in a paravirtualization setting).

              The ARCH_SET_CPUID setting is preserved across fork(2) and
              clone(2) but reset to the default (i.e., cpuid enabled) on

       ARCH_GET_CPUID (since Linux 4.12)
              Return the setting of the flag manipulated by ARCH_SET_CPUID as
              the result of the system call (1 for enabled, 0 for disabled).
              addr is ignored.

       Subfunctions for x86-64 only are:

              Set the 64-bit base for the FS register to addr.

              Return the 64-bit base value for the FS register of the calling
              thread in the unsigned long pointed to by addr.

              Set the 64-bit base for the GS register to addr.

              Return the 64-bit base value for the GS register of the calling
              thread in the unsigned long pointed to by addr.

       On success, arch_prctl() returns 0; on error, -1 is returned, and errno
       is set to indicate the error.

       EFAULT addr points to an unmapped address or is outside the process
              address space.

       EINVAL code is not a valid subcommand.

       EPERM  addr is outside the process address space.

       ENODEV ARCH_SET_CPUID was requested, but the underlying hardware does not
              support CPUID faulting.

       arch_prctl() is a Linux/x86-64 extension and should not be used in
       programs intended to be portable.

       arch_prctl() is supported only on Linux/x86-64 for 64-bit programs

       The 64-bit base changes when a new 32-bit segment selector is loaded.

       ARCH_SET_GS is disabled in some kernels.

       Context switches for 64-bit segment bases are rather expensive.  As an
       optimization, if a 32-bit TLS base address is used, arch_prctl() may use
       a real TLS entry as if set_thread_area(2) had been called, instead of
       manipulating the segment base register directly.  Memory in the first
       2 GB of address space can be allocated by using mmap(2) with the
       MAP_32BIT flag.

       Because of the aforementioned optimization, using arch_prctl() and
       set_thread_area(2) in the same thread is dangerous, as they may overwrite
       each other's TLS entries.

       As of version 2.7, glibc provides no prototype for arch_prctl().  You
       have to declare it yourself for now.  This may be fixed in future glibc

       FS may be already used by the threading library.  Programs that use
       ARCH_SET_FS directly are very likely to crash.

       mmap(2), modify_ldt(2), prctl(2), set_thread_area(2)

       AMD X86-64 Programmer's manual

       This page is part of release 5.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2020-04-11                      ARCH_PRCTL(2)