I believe the REQUIRED_MASK aproach was taken so that it was
easier to consult in assembly (arch/x86/kernel/verify_cpu.S).
DISABLED_MASK does not have the same restriction, but I
implemented it the same way for consistency.

We have a REQUIRED_MASK... which does two things:
1. Keeps a list of cpuid bits to check in very early boot and
   refuse to boot if those are not present.
2. Consulted during cpu_has() checks, which allows us to
   optimize out things at compile-time.  In other words, if we
   *KNOW* we will not boot with the feature off, then we can
   safely assume that it will be present forever.

But, we don't have a similar mechanism for CPU features which
may be present but that we know we will not use.  We simply
use our existing mechanisms to repeatedly check the status of
the bit at runtime (well, the alternatives patching helps here
but it does not provide compile-time optimization).

Adding a feature to disabled-features.h allows the bit to be
checked via a new macro: cpu_feature_enabled().  Note that
for features in DISABLED_MASK, checks with this macro have
all of the benefits of an #ifdef.  Before, we would have done
this in a header:

#ifdef CONFIG_X86_INTEL_MPX
#define cpu_has_mpx cpu_has(X86_FEATURE_MPX)
#else
#define cpu_has_mpx 0
#endif

and this in the code:

	if (cpu_has_mpx)
		do_some_mpx_thing();

Now, just add your feature to DISABLED_MASK and you can do this
everywhere, and get the same benefits you would have from
#ifdefs:

	if (cpu_feature_enabled(X86_FEATURE_MPX))
		do_some_mpx_thing();

We need a new function and *not* a modification to cpu_has()
because there are cases where we actually need to check the CPU
itself, despite what features the kernel supports.  The best
example of this is a hypervisor which has no control over what
features its guests are using and where the guest does not depend
on the host for support.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: http://lkml.kernel.org/r/20140911211513.9E35E931@viggo.jf.intel.com


Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>