From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757502AbaGBT1T (ORCPT ); Wed, 2 Jul 2014 15:27:19 -0400 Received: from mail.skyhub.de ([78.46.96.112]:50560 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754452AbaGBT1S (ORCPT ); Wed, 2 Jul 2014 15:27:18 -0400 Date: Wed, 2 Jul 2014 21:27:09 +0200 From: Borislav Petkov To: Dave Hansen Cc: x86@kernel.org, hpa@zytor.com, qiaowei.ren@intel.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/4] x86: introduce disabled-features Message-ID: <20140702192709.GK1318@pd.tnic> References: <20140702162925.B07A3AB9@viggo.jf.intel.com> <20140702162928.B3BBEB26@viggo.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20140702162928.B3BBEB26@viggo.jf.intel.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 02, 2014 at 09:29:28AM -0700, Dave Hansen wrote: > > From: Dave Hansen > > 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 Acked-by: Borislav Petkov ... > +/* > + * 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. --