Documentation/x86: Add documentation for using dynamic XSTATE features

mainline inclusion
from mainline-v5.16-rc1
commit d7a9590f
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I590ZC


CVE: NA

Intel-SIG: commit d7a9590f Documentation/x86: Add documentation for using dynamic XSTATE features.

--------------------------------

Explain how dynamic XSTATE features can be enabled via the
architecture-specific prctl() along with dynamic sigframe size and
first use trap handling.

Fix:

Documentation/x86/xstate.rst:15: WARNING: Title underline too short.

as reported by Stephen Rothwell <sfr@canb.auug.org.au>

Originally-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211026091157.16711-1-chang.seok.bae@intel.com


Signed-off-by: Lin Wang <lin.x.wang@intel.com>
Signed-off-by: Aichun Shi <aichun.shi@intel.com>