%cr4 is supposed to reflect a set of features into which the operating
system is opting in.  If the BIOS or bootloader leaks bits here, this
is not desirable.  Consider a bootloader passing in %cr4.pae set to a
legacy paging kernel, for example -- it will not have any immediate
effect, but the kernel would crash when turning paging on.

A similar argument applies to %eflags, and since we have to look for
%eflags.id being settable we can use a sequence which clears %eflags
as a side effect.

Note that we already do this for x86-64.
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/1348529239-17943-1-git-send-email-hpa@linux.intel.com