linux-kernel - Re: [PATCH v3 4/5] x86/mm: optimize static

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Date:   Tue, 4 Sep 2018 14:22:03 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Bin Yang <bin.yang@...el.com>
cc:     mingo@...nel.org, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        dave.hansen@...el.com, mark.gross@...el.com
Subject: Re: [PATCH v3 4/5] x86/mm: optimize static_protection() by using
 overlap()

On Tue, 21 Aug 2018, Bin Yang wrote:
>  
> +static inline bool
> +overlap(unsigned long start1, unsigned long end1,
> +		unsigned long start2, unsigned long end2)
> +{
> +	/* Is 'start2' within area 1? */
> +	if (start1 <= start2 && end1 > start2)
> +		return true;
> +
> +	/* Is 'start1' within area 2? */
> +	if (start2 <= start1 && end2 > start1)
> +		return true;
> +
> +	return false;

>  static inline unsigned long highmap_start_pfn(void)
> @@ -293,7 +308,7 @@ static void cpa_flush_array(unsigned long *start, int numpages, int cache,
>   * checks and fixes these known static required protection bits.
>   */
>  static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
> -				   unsigned long pfn)
> +				   unsigned long len, unsigned long pfn)
>  {
>  	pgprot_t forbidden = __pgprot(0);
>  
> @@ -302,7 +317,9 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
>  	 * PCI BIOS based config access (CONFIG_PCI_GOBIOS) support.
>  	 */
>  #ifdef CONFIG_PCI_BIOS
> -	if (pcibios_enabled && within(pfn, BIOS_BEGIN >> PAGE_SHIFT, BIOS_END >> PAGE_SHIFT))
> +	if (pcibios_enabled &&
> +	    overlap(pfn, pfn + PFN_DOWN(len),
> +		    PFN_DOWN(BIOS_BEGIN), PFN_DOWN(BIOS_END)))

This is completely unreadable and aside of that it is wrong. You cannot do
an overlap check with the following constraints:

   	   range1_end = range1_start + size;
   	   range2_end = range2_start + size;

See the definition of BIOS_END. It's 0x100000, i.e. 1MB, so the following
overlap check will give you the false result:

	overlap(256, 258, 0x000a0000 >> 12, 0x0010000 >> 12)

because

	0x0010000 >> 12 = 256

ergo will overlap return true. All of your overlap checks are broken.

Oh well.

Thanks,

	tglx