Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965105AbbLHPQh (ORCPT ); Tue, 8 Dec 2015 10:16:37 -0500 Received: from www.linutronix.de ([62.245.132.108]:50130 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932870AbbLHPQf (ORCPT ); Tue, 8 Dec 2015 10:16:35 -0500 Date: Tue, 8 Dec 2015 16:15:29 +0100 (CET) From: Thomas Gleixner To: Dave Hansen cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, dave.hansen@linux.intel.com Subject: Re: [PATCH 10/34] x86, pkeys: arch-specific protection bitsy In-Reply-To: <20151204011438.E50D1498@viggo.jf.intel.com> Message-ID: References: <20151204011424.8A36E365@viggo.jf.intel.com> <20151204011438.E50D1498@viggo.jf.intel.com> User-Agent: Alpine 2.11 (DEB 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1988 Lines: 71 Dave, On Thu, 3 Dec 2015, Dave Hansen wrote: > > +static inline int vma_pkey(struct vm_area_struct *vma) Shouldn't this return something unsigned? > +{ > + u16 pkey = 0; > +#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS > + unsigned long vma_pkey_mask = VM_PKEY_BIT0 | VM_PKEY_BIT1 | > + VM_PKEY_BIT2 | VM_PKEY_BIT3; > + /* > + * ffs is one-based, not zero-based, so bias back down by 1. > + */ > + int vm_pkey_shift = __builtin_ffsl(vma_pkey_mask) - 1; Took me some time to figure out that this will resolve to a compile time constant (hopefully). Is there a reason why we don't have a VM_PKEY_SHIFT constant in the header file which makes that code just simple and intuitive? > + /* > + * gcc generates better code if we do this rather than: > + * pkey = (flags & mask) >> shift > + */ > + pkey = (vma->vm_flags >> vm_pkey_shift) & > + (vma_pkey_mask >> vm_pkey_shift); My gcc (4.9) does it the other way round for whatever reason. I really prefer to have this as simple as: #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS #define VM_PKEY_MASK (VM_PKEY_BIT0 | VM_PKEY_BIT1 | VM_PKEY_BIT2 | VM_PKEY_BIT3) #define VM_PKEY_SHIFT #else #define VM_PKEY_MASK 0UL #define VM_PKEY_SHIFT 0 #endif static inline unsigned int vma_pkey(struct vm_area_struct *vma) { return (vma->vm_flags & VM_PKEY_MASK) >> VM_PKEY_SHIFT; } or #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS #define VM_PKEY_MASK (VM_PKEY_BIT0 | VM_PKEY_BIT1 | VM_PKEY_BIT2 | VM_PKEY_BIT3) #define VM_PKEY_SHIFT static inline unsigned int vma_pkey(struct vm_area_struct *vma) { return (vma->vm_flags & VM_PKEY_MASK) >> VM_PKEY_SHIFT; } #else static inline unsigned int vma_pkey(struct vm_area_struct *vma) { return 0; } #endif Hmm? tglx -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/