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    SubjectRe: [Xen-devel] Re: [PATCH 00 of 36] x86/paravirt: groundwork for 64-bit Xen support

    * Jeremy Fitzhardinge <jeremy@goop.org> wrote:

    > Ingo Molnar wrote:
    >> -tip auto-testing found pagetable corruption (CPA self-test failure):
    >>
    >> [ 32.956015] CPA self-test:
    >> [ 32.958822] 4k 2048 large 508 gb 0 x 2556[ffff880000000000-ffff88003fe00000] miss 0
    >> [ 32.964000] CPA ffff88001d54e000: bad pte 1d4000e3
    >> [ 32.968000] CPA ffff88001d54e000: unexpected level 2
    >> [ 32.972000] CPA ffff880022c5d000: bad pte 22c000e3
    >> [ 32.976000] CPA ffff880022c5d000: unexpected level 2
    >> [ 32.980000] CPA ffff8800200ce000: bad pte 200000e3
    >> [ 32.984000] CPA ffff8800200ce000: unexpected level 2
    >> [ 32.988000] CPA ffff8800210f0000: bad pte 210000e3
    >>
    >> config and full log can be found at:
    >>
    >> http://redhat.com/~mingo/misc/config-Mon_Jun_30_11_11_51_CEST_2008.bad
    >> http://redhat.com/~mingo/misc/log-Mon_Jun_30_11_11_51_CEST_2008.bad
    >>
    >> i've pushed that tree out into tip/tmp.xen-64bit.Mon_Jun_30_11_11. The
    >> only new item in that tree over a well-tested base is x86/xen-64bit, so
    >> i've taken it out again.
    >>
    >
    > Phew. OK, I've worked this out. Short version is that's it's a false
    > alarm, and there was no real failure here. Long version:
    >
    > * I changed the code to create the physical mapping pagetables to
    > reuse any existing mapping rather than replace it. Specifically,
    > reusing an pud pointed to by the pgd caused this symptom to appear.
    > * The specific PUD being reused is the one created statically in
    > head_64.S, which creates an initial 1GB mapping.
    > * That mapping doesn't have _PAGE_GLOBAL set on it, due to the
    > inconsistency between __PAGE_* and PAGE_*.
    > * The CPA test attempts to clear _PAGE_GLOBAL, and then checks to
    > see that the resulting range is 1) shattered into 4k pages, and 2)
    > has no _PAGE_GLOBAL.
    > * However, since it didn't have _PAGE_GLOBAL on that range to start
    > with, change_page_attr_clear() had nothing to do, and didn't
    > bother shattering the range,
    > * resulting in the reported messages
    >
    > The simple fix is to set _PAGE_GLOBAL in level2_ident_pgt.
    >
    > An additional fix to make CPA testing more robust by using some other
    > pagetable bit (one of the unused available-to-software ones). This
    > would solve spurious CPA test warnings under Xen which uses _PAGE_GLOBAL
    > for its own purposes (ie, not under guest control).
    >
    > Also, we should revisit the use of _PAGE_GLOBAL in asm-x86/pgtable.h,
    > and use it consistently, and drop MAKE_GLOBAL. The first time I
    > proposed it it caused breakages in the very early CPA code; with luck
    > that's all fixed now.
    >
    > Anyway, the simple fix below. [...]

    great - i've applied your fix and re-integrated x86/xen-64bit, it's
    under testing now. (no problems so far)

    > [...] I'll put together RFC patches for the other suggestions. I also
    > split the originating patch into tiny, tiny bisectable pieces.

    cool! :)

    Ingo


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