If a processor implementation discern that a processor state component is in
its initialized state, it may modify the corresponding bit in the
xsave header.xstate_bv as '0'. State in the memory layout setup by 'xsave'
will be consistent with the bit values in the header.

During signal handling, legacy applications may change the FP/SSE bits
in the sigcontext memory layout without touching the FP/SSE header bits
in the xsave header. So always set FP/SSE bits in the xsave header
while saving the sigcontext state to the user space. During signal return,
this will enable the kernel to capture any changes to the FP/SSE bits by the
legacy applications which don't touch xsave headers.

xsave aware apps can change the xstate_bv in the xsave header aswell
as change any contents in the memory layout. xrestor as part of sigreturn
will capture all the changes.
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>