[PATCH v4 1/4] Documentation/x86: Explain the purpose for dynamic features

["Chang S. Bae" <chang.seok.bae@intel.com>]

This summary will help to guide the proper use of the enabling model.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-doc@vger.kernel.org
---
Changes from v3:
* Add as a new patch (Tony Luck).
---
 Documentation/x86/xstate.rst | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/Documentation/x86/xstate.rst b/Documentation/x86/xstate.rst
index 5cec7fb558d6..2577b28ad942 100644
--- a/Documentation/x86/xstate.rst
+++ b/Documentation/x86/xstate.rst
@@ -11,6 +11,20 @@ are enabled by XCR0 as well, but the first use of related instruction is
 trapped by the kernel because by default the required large XSTATE buffers
 are not allocated automatically.
 
+The purpose for dynamic features
+--------------------------------
+
+ - Legacy userspace libraries have hard-coded sizes for an alternate signal
+   stack. With the arch_prctl() options, the signal frame beyond AVX-512
+   and PKRU will not be written by old programs as they are prevented from
+   using dynamic features. Then, the small signal stack will be compatible
+   on systems that support dynamic features.
+
+ - Modern server systems are consolidating more applications to share the
+   CPU resource. The risk of applications interfering with each other is
+   growing. The controllability on the resource trends to be more
+   warranted. Thus, this permission mechanism will be useful for that.
+
 Using dynamically enabled XSTATE features in user space applications
 --------------------------------------------------------------------
 
-- 
2.17.1