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    SubjectRe: [Xen-devel] incorrect layout of globals from head_64.S during kexec boot
    On Tue, Jul 10, Konrad Rzeszutek Wilk wrote:

    > On Tue, Jul 10, 2012 at 05:23:08PM +0200, Olaf Hering wrote:
    > > I was not thinking of statically allocated pages but some new concept of
    > > allocating such shared pages. Shouldnt there be some dedicated area in
    > > the E820 table which has to be used during the whole life time of the
    > > guest?
    >
    > Not that I can see. But I don't see why that could not be added? Perhaps
    > the HVM loader can make it happen? But then how would it tell the kernel
    > that this E820_RESERVED is the shared_info one. Not the other ones..

    Maybe just use a new E820 type for this sort of thing? Its just the
    question wether some other OS can cope with an unknown type. From my
    reading of the e820 related code a region with an unknown type is just
    ignored.

    > > Are there more shared areas or is it just the shared info page?
    > >
    > > > And I am kind of worried that moving it to the .data section won't
    > > > be completly safe - as the decompressor might blow away that part too.
    > >
    > > The decompressor may just clear the area, but since there is no way to
    > > tell where the shared pages are its always a risk to allocate them at
    > > compile time.
    >
    > Yeah, and with the hypervisor potentially still updating the "old"
    > MFN before the new kernel has registered the new MFN, we can end up
    > corrupting the new kernel. Ouch.
    >
    > Would all of these issues disappear if the hypervisor had a hypercall
    > that would stop updating the shared info? or just deregister the MFN?
    > What if you ripped the GMFN out using 'decrease_reservation' hypercall?
    > Would that eliminate the pesky GMFN?

    I'm not sure, most likely the gfn will just disappear from the guest,
    like a ballooned page disappears. Accessing it will likely cause a
    crash.

    Olaf


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