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    SubjectRe: [PATCH v4 2/3] x86/kexec/64: Error out if try to jump to old 4-level kernel from 5-level kernel
    On 05/09/19 at 09:36am, Baoquan He wrote:
    > If the running kernel has 5-level paging activated, the 5-level paging
    > mode is preserved across kexec. If the kexec'ed kernel does not contain
    > support for handling active 5-level paging mode in the decompressor, the
    > decompressor will crash with #GP.
    >
    > Prevent this situation at load time. If 5-level paging is active, check the
    > xloadflags whether the kexec kernel can handle 5-level paging at least in
    > the decompressor. If not, reject the load attempt and print out error
    > message.
    >
    > Signed-off-by: Baoquan He <bhe@redhat.com>
    > Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    > ---
    > arch/x86/kernel/kexec-bzimage64.c | 5 +++++
    > 1 file changed, 5 insertions(+)
    >
    > diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
    > index 22f60dd26460..858cc892672f 100644
    > --- a/arch/x86/kernel/kexec-bzimage64.c
    > +++ b/arch/x86/kernel/kexec-bzimage64.c
    > @@ -321,6 +321,11 @@ static int bzImage64_probe(const char *buf, unsigned long len)
    > return ret;
    > }
    >
    > + if (!(header->xloadflags & XLF_5LEVEL) && pgtable_l5_enabled()) {
    > + pr_err("Can not jump to old 4-level kernel from 5-level kernel.\n");

    Check the old thread again, found Thomas also had comment about this
    error message. Just forgot updating it. Will update it with a new one.

    > + return ret;
    > + }
    > +
    > /* I've got a bzImage */
    > pr_debug("It's a relocatable bzImage64\n");
    > ret = 0;
    > --
    > 2.17.2
    >

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