On Wed, Jul 02, 2014 at 09:29:28AM -0700, Dave Hansen wrote: > > From: Dave Hansen <dave.han...@linux.intel.com> > > 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.han...@linux.intel.com>
Acked-by: Borislav Petkov <b...@suse.de> ... > +/* > + * Make sure to add features to the correct mask Hehe, good. :-) > + */ > +#define DISABLED_MASK0 0 > +#define DISABLED_MASK1 0 > +#define DISABLED_MASK2 0 > +#define DISABLED_MASK3 0 > +#define DISABLED_MASK4 0 > +#define DISABLED_MASK5 0 > +#define DISABLED_MASK6 0 > +#define DISABLED_MASK7 0 > +#define DISABLED_MASK8 0 > +#define DISABLED_MASK9 0 > + > +#endif /* _ASM_X86_DISABLED_FEATURES_H */ > _ > -- Regards/Gruss, Boris. Sent from a fat crate under my desk. Formatting is fine. -- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/