ARCH_PRCTL(2)             (2020-04-11)              ARCH_PRCTL(2)

     NAME
          arch_prctl - set architecture-specific thread state

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

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

     DESCRIPTION
          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 sig-
               nal.  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 execve(2).

          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:

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

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

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

          ARCH_GET_GS

     Page 1                        Linux             (printed 5/24/22)

     ARCH_PRCTL(2)             (2020-04-11)              ARCH_PRCTL(2)

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

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

     ERRORS
          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 hard-
               ware does not support CPUID faulting.

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

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

          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 expen-
          sive.  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 versions.

          FS may be already used by the threading library.  Programs

     Page 2                        Linux             (printed 5/24/22)

     ARCH_PRCTL(2)             (2020-04-11)              ARCH_PRCTL(2)

          that use ARCH_SET_FS directly are very likely to crash.

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

          AMD X86-64 Programmer's manual

     COLOPHON
          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 https://www.kernel.org/doc/man-pages/.

     Page 3                        Linux             (printed 5/24/22)