Re: [PATCH v3 4/5] x86/mm: optimize static_protection() by using overlap()

Thomas Gleixner
2018-09-04T12:22:19Z

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