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    SubjectRe: [Xen-devel] incorrect layout of globals from head_64.S during kexec boot
    On 13/07/2012 21:20, "Olaf Hering" <> wrote:

    > 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 :)
    > Keir,
    > 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.

    So pfn A now points at shared_info, and mfn M is lost (freed back to Xen).
    Xen_vcpu doesn't come into it, you'd have that mapped at yet another pfn.

    > 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
    > 0xff.

    Don't really understand this. After the XENMAPSPACE_shared_info_call:
    * PFN B points at shared_info, mfn M_B it previously mapped is lost (freed
    back to Xen).
    * PFN A maps nothing, reads return all-1s.

    Yes, obviously you can't atomically update the mapping of shinfo from A->B,
    ad update your pointer in the kernel at exactly the same time. Presumably
    you do this early during boot, or late during kexec, or otherwise at a time
    when other processors are not expected to touch shinfo.

    > 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.

    That is correct.

    > What do you think, is that race real?

    I suppose it is. I didn't imagine it would be a troublesome one though.

    -- Keir

    > Olaf

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