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    SubjectRe: [Xen-devel] Re: [PATCH 1 of 7] x86: add _PAGE_IOMAP pte flag for IO mappings
    Keir Fraser wrote:
    > That's basically what our _PAGE_IO flag (in our old Linux patchset) means.
    > We use it to cause pte_pfn() to return an invalid pfn and hence avoid
    > reference counting that way. Since kernel mappings are never reference
    > counted (I think?) perhaps we could use _PAGE_SPECIAL even if it is
    > restricted to use on user mappings.

    Well, _PAGE_IOMAP's most important semantic from Xen's perspective is
    that the frame number is considered to already be an MFN and so isn't
    converted. It may be that _PAGE_SPECIAL is also useful for its "no
    refcount" properties, but we could set both in that case.

    At present we can't set _PAGE_SPECIAL on kernel mappings because we
    assume its usermode only, and its shared with _PAGE_CPA_TEST which is
    kernel-only. Most _PAGE_IOMAP mappings are in kernel space, but I
    wouldn't discount user mappings if we end up allowing direct access to
    passthrough devices.


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