x86/xsaves: Add a kernel parameter noxsaves to disable xsaves/xrstors

This patch adds a kernel parameter noxsaves to disable xsaves/xrstors feature.
The kernel will fall back to use xsaveopt and xrstor to save and restor
xstates. By using this parameter, xsave area occupies more memory because
standard form of xsave area in xsaveopt/xrstor occupies more memory than
compacted form of xsave area.

This patch adds a description of the kernel parameter noxsaveopt in doc.
The code to support the parameter noxsaveopt has been in the kernel before.
This patch just adds the description of this parameter in the doc.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1401387164-43416-4-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

Documentation/kernel-parameters.txt

@@ -2124,6 +2124,21 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			and restore using xsave. The kernel will fallback to
 			enabling legacy floating-point and sse state.
 
+	noxsaveopt	[X86] Disables xsaveopt used in saving x86 extended
+			register states. The kernel will fall back to use
+			xsave to save the states. By using this parameter,
+			performance of saving the states is degraded because
+			xsave doesn't support modified optimization while
+			xsaveopt supports it on xsaveopt enabled systems.
+
+	noxsaves	[X86] Disables xsaves and xrstors used in saving and
+			restoring x86 extended register state in compacted
+			form of xsave area. The kernel will fall back to use
+			xsaveopt and xrstor to save and restore the states
+			in standard form of xsave area. By using this
+			parameter, xsave area per process might occupy more
+			memory on xsaves enabled systems.
+
 	eagerfpu=	[X86]
 			on	enable eager fpu restore
 			off	disable eager fpu restore