Commit:     1ce70c4fac3c3954bd48c035f448793867592bc0
Parent:     db342d216ba9e060d8c5501eefc1d0a789c9e711
Author:     Ahmed S. Darwish <[EMAIL PROTECTED]>
AuthorDate: Sun Feb 24 17:55:15 2008 +0200
Committer:  Ingo Molnar <[EMAIL PROTECTED]>
CommitDate: Tue Feb 26 12:55:49 2008 +0100

    x86/lguest: fix pgdir pmd index calculation
    Hi all,
    Beginning from commits close to v2.6.25-rc2, running lguest always oopses
    the host kernel. Oops is at [1].
    Bisection led to the following commit:
    commit 37cc8d7f963ba2deec29c9b68716944516a3244f
        x86/early_ioremap: don't assume we're using swapper_pg_dir
        At the early stages of boot, before the kernel pagetable has been
        fully initialized, a Xen kernel will still be running off the
        Xen-provided pagetables rather than swapper_pg_dir[].  Therefore,
        readback cr3 to determine the base of the pagetable rather than
        assuming swapper_pg_dir[].
     static inline pmd_t * __init early_ioremap_pmd(unsigned long addr)
    -   pgd_t *pgd = &swapper_pg_dir[pgd_index(addr)];
    +   /* Don't assume we're using swapper_pg_dir at this point */
    +   pgd_t *base = __va(read_cr3());
    +   pgd_t *pgd = &base[pgd_index(addr)];
        pud_t *pud = pud_offset(pgd, addr);
        pmd_t *pmd = pmd_offset(pud, addr);
    Trying to analyze the problem, it seems on the guest side of lguest,
    %cr3 has a different value from &swapper_pg-dir (which
    is AFAIK fine on a pravirt guest):
    Putting some debugging messages in early_ioremap_pmd:
    /* Appears 3 times */
    [    0.000000] ***************************
    [    0.000000] __va(%cr3) = c0000000, &swapper_pg_dir = c02cc000
    [    0.000000] ***************************
    After 8 hours of debugging and staring on lguest code, I noticed something
    strange in paravirt_ops->set_pmd hypercall invocation:
    static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
        *pmdp = pmdval;
        lazy_hcall(LHCALL_SET_PMD, __pa(pmdp)&PAGE_MASK,
                   (__pa(pmdp)&(PAGE_SIZE-1))/4, 0);
    The first hcall parameter is global pgdir which looks fine. The second
    parameter is the pmd index in the pgdir which is suspectful.
    AFAIK, calculating the index of pmd does not need a divisoin over four.
    Removing the division made lguest work fine again . Patch is at [2].
    I am not sure why the division over four existed in the first place. It
    seems bogus, maybe the Xen patch just made the problem appear ?
    [2]: The patch:
    [PATCH] lguest: fix pgdir pmd index cacluation
    Remove an error in index calculation which leads to removing
    a not existing shadow page table (leading to a Null dereference).
    Signed-off-by: Ahmed S. Darwish <[EMAIL PROTECTED]>
    Signed-off-by: Ingo Molnar <[EMAIL PROTECTED]>
 arch/x86/lguest/boot.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index 1e613fb..cccb38a 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -481,7 +481,7 @@ static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
        *pmdp = pmdval;
        lazy_hcall(LHCALL_SET_PMD, __pa(pmdp)&PAGE_MASK,
-                  (__pa(pmdp)&(PAGE_SIZE-1))/4, 0);
+                  (__pa(pmdp)&(PAGE_SIZE-1)), 0);
 /* There are a couple of legacy places where the kernel sets a PTE, but we
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