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    SubjectRe: [Bug Fix]: Do 32-bit table calculations in pre-processor
    Date
    On Fri July 3 2009, Jeremy Fitzhardinge wrote:
    > [ Missed some recipients. ]
    >
    > On 07/03/09 11:14, Michael S. Zick wrote:
    > > Here is one I have found useful in my VIA processor bug hunting:
    > >
    > > diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
    > > index 3068388..2303d86 100644
    > > --- a/arch/x86/kernel/head_32.S
    > > +++ b/arch/x86/kernel/head_32.S
    > > @@ -61,7 +61,7 @@
    > >
    > > /* Enough space to fit pagetables for the low memory linear map */
    > > MAPPING_BEYOND_END = \
    > > - PAGE_TABLE_SIZE(((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT) << PAGE_SHIFT
    > > + PAGE_TABLE_SIZE((1<<20) - (__PAGE_OFFSET >> PAGE_SHIFT)) << PAGE_SHIFT
    > >
    > >
    >
    > These calculations are performed by the assembler, not the
    > preprocessor. I think this transformation looks correct (in that its an
    > identity with the original), but my understanding is that the assembler
    > does it calculations in arbitrary precision, so there's no need to worry
    > about limiting the arithmetic to 32-bits.
    >
    > > /*
    > > * Worst-case size of the kernel mapping we need to make:
    > >
    > > = = =
    > >
    > > Before:
    > >
    > > #5 [0000010000 - 0000011000] PGTABLE ==> [0000010000 - 0000011000]
    > > #6 [0000011000 - 0000015000] BOOTMAP ==> [0000011000 - 0000015000]
    > >
    > > After:
    > >
    > > #5 [0000010000 - 000007d000] PGTABLE ==> [0000010000 - 000007d000]
    > > #6 [000007d000 - 0000081000] BOOTMAP ==> [000007d000 - 0000081000]
    > >
    > > Someone who knows mm check which is the reasonable value please.
    > >
    >
    >
    > The PGTABLE reservation seems much too big. I think 1 page should be
    > sufficient for a system with large pages. Even if not, 0x6d000 is way
    > too large. And they symptoms of failing to reserve the initial
    > pagetable are pretty non-subtle.
    >

    Random system halts, deadlocks with interrupts disabled?
    Yup, that sounds familiar.

    If I ever get more than a stopped machine with a glowing power light;
    I will be certain to share.

    Mike
    > J
    >
    >




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