linux-kernel - Re: [PATCH 10/34] x86, pkeys: arch-specific protection bitsy

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Message-ID: <alpine.DEB.2.11.1512081523180.3595@nanos>
Date:	Tue, 8 Dec 2015 16:15:29 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dave Hansen <dave@...1.net>
cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
	dave.hansen@...ux.intel.com
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
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