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    SubjectRe: [PATCH] paravirt.h
    Andi Kleen wrote:
    > Zachary Amsden <zach@vmware.com> writes:
    >
    >> That is a really nasty problem. You need a synchronization primitive
    >> which guarantees a flat stack, so you can't do it in the interrupt
    >> handler as I have tried to do. I'll bang my head on it awhile. In
    >> the meantime, were there ever any solutions to the syscall patching
    >> problem that might lend me a clue as to what to do (or not to do, or
    >> impossible?).
    >>
    >
    > stop_machine_run() solves the problem I think. It is currently not
    > exported though. I don't think there's anything in there that couldn't
    > be reimplemented in a module, but then we could also just export it
    > if there's a useful user.
    >

    Well, I don't think anything is sufficient for a preemptible kernel. I
    think that's just plain not going to work. You could have a kernel
    thread that got preempted in a paravirt-op patch point, and making all
    the patch points non-preempt is probably a non-starter (either +12 bytes
    each or no native inlining). Finding out after the fact that you have a
    kernel thread that was preempted in a patch point is very hard work, but
    it is possible. The fixing it up is where you need to take liberties
    with reality.

    stop_machine_run() is almost what I want, but even that is not
    sufficient. You also need to disable NMIs and debug traps, which is
    pretty hairy, but doable. The problem with stop_machine_run() is that I
    don't just want the kernel to halt running on remote CPUs, I want the
    kernel on all CPUs to actually do something simultaneously - the entry
    into paravirt mode requires a hypervisor call on each CPU, and
    stop_machine() doesn't provide a facility to fire a callback on each CPU
    from the stopmachine state.

    Since this code is so rather, um, custom, I was going to reimplement
    stop_machine in the module.

    Zach
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