x86/xsaves: Use xsave/xrstor for saving and restoring user space context

x86/xsaves: Use xsave/xrstor for saving and restoring user space context

We use legacy xsave/xrstor to save and restore standard form of xsave area
in user space context. No xsaveopt or xsaves is used here for two reasons.

First, we don't want to use modified optimization which is implemented in
xsaveopt and xsaves because xrstor/xrstors might track a wrong user space
application.

Secondly, we don't use compacted format of xsave area for backward
compatibility because legacy user space applications only don't understand
the compacted format of the xsave area.

Using standard form of the xsave area may allocate more memory for
user context than compacted form, but preserves compatibility with
legacy applications.  Furthermore, even with holes, the relevant cache
lines don't get touched and thus the performance impact is limited.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1401387164-43416-11-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>