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    SubjectRe: [PATCH] x86: use pgd accessors when cloning a pgd range.
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    On Wed, 2010-10-27 at 18:51 +0100, Jeremy Fitzhardinge wrote:
    > On 10/27/2010 10:42 AM, H. Peter Anvin wrote:

    > > Ian's message just mentioned "a failure" and never described in any way what
    > > kind of "failure" it was.
    >
    > It would be a pagefault from Xen preventing a direct write to the pgd
    > level of an active pagetable. At the point in setup_arch() where it
    > does the first clone_pgd_range() we're already running on swapper_pg_dir
    > and the copy from initial_page_table is outright wrong.

    I'd missed that aspect, yes the contents of initial_page_table are
    wrong. I'm not sure now how my patch to clone_pgd_range even made a
    difference...

    > As Ian suggests, we could switch Xen to use initial_page_table at boot
    > then move to swapper_pg_dir in the same way native does.

    Accidentally did that in private mail, for everyone else the gory
    details are:

    > xen_setup_kernel_pagetable operates on initial_page_table instead of
    > swapper_pg_dir. We do not pin initial_page_table apart from the
    > implicit
    > one from writing it to cr3 (it is necessarily r/o though).
    >
    > So we enter setup_arch running on initial_page_table and with
    > swapper_pg_dir mapped r/w, which is how it looks on native too. So at
    > this point the:
    > clone_pgd_range(swapper_pg_dir + KERNEL_PGD_BOUNDARY,
    > initial_page_table + KERNEL_PGD_BOUNDARY,
    > KERNEL_PGD_PTRS);
    > works just fine and we reach:
    > load_cr3(swapper_pg_dir);
    >
    > Then in __xen_write_cr3 we notice the first attempt to switch to
    > swapper_pg_dir and only at that point do we make it r/o and pin it.
    > When
    > we then actually do the switch to swapper_pg_dir that releases the
    > implicit pin on initial_page_table so we can make it r/o again.
    >
    > The later on we reach the:
    > clone_pgd_range(initial_page_table + KERNEL_PGD_BOUNDARY,
    > swapper_pg_dir + KERNEL_PGD_BOUNDARY,
    > KERNEL_PGD_PTRS);
    > which will succeed because initial_page_table is r/w again.
    >
    > We don't care about the pin on initial_page_table because we should
    > never need it again.

    Ian.



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