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    SubjectRe: [patch 1/2] x86_64 page fault NMI-safe
    On 07/14/2010 03:37 PM, Linus Torvalds wrote:
    > I think the %rip check should be pretty simple - exactly because there
    > is only a single point where the race is open between that 'mov' and
    > the 'iret'. So it's simpler than the (similar) thing we do for
    > debug/nmi stack fixup for sysenter that has to check a range.
    >
    > The only worry is if that crazy paravirt code wants to paravirtualize
    > the iretq. Afaik, paravirt does that exactly because they screw up
    > iret handling themselves. Maybe we could stop doing that stupid iretq
    > paravirtualization, and just tell the paravirt people to do the same
    > thing I propose, and just allow nesting.
    >

    We screw around with iret because there's a separate interrupt mask flag
    which can't be set atomically with respect to a stack/ring change (well,
    there's more to it, but I won't confuse matters).

    But it only really matters if the PV guest can also get NMI-like
    interrupts. While Xen supports NMI for PV guests, we don't use it much
    and I haven't looked into implementing support for it yet.

    J


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