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

    > On 10/07/2012 19:09, "Olaf Hering" <> wrote:
    > > 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.
    > Best thing to do, is possible, is map the shared-info page in the
    > xen-platform pci device's BAR memory range. Then it will not conflict with
    > any RAM.
    > If you do map it over the top of an existing RAM page, you will have to
    > repopulate that RAM page before kexec, using populate_physmap hypercall. The
    > good news is that the populate_physmap hypercall will have the side effect
    > of unmapping the shared-info page, reayd to be mapped wherever the new
    > kernel would like it to reside :)


    is this a safe thing to do in a SMP guest?
    If arch/x86/xen/enlighten.c:xen_hvm_init_shared_info() allocates a page
    (backed by mfn M and pfn A) and assigns *HYPERVISOR_shared_info and
    *xen_vcpu then everything will reference these pointers.

    If drivers/xen/platform-pci.c:platform_pci_init would also do a
    XENMAPSPACE_shared_info call with pfn B, isnt there a small window where
    pfn A is not backed by a mfn because mfn M is now connected to pfn C? As
    a result other code paths which access *HYPERVISOR_shared_info and
    *xen_vcpu between the hypercall and the update of the pointers will read

    If I read the hypercall code of XENMEM_add_to_physmap correctly the mfn
    backing *HYPERVISOR_shared_info will remain the same, so there is no need
    to copy data from the old to the new *HYPERVISOR_shared_info.

    What do you think, is that race real?


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