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
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