Messages in this thread | | | Date | Tue, 8 Dec 2015 16:15:29 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH 10/34] x86, pkeys: arch-specific protection bitsy |
| |
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
| |