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    SubjectRe: [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


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