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    SubjectRe: [PATCH v3] enable x2APIC without interrupt remapping under KVM
    On 06/30/2009 10:08 PM, Eric W. Biederman wrote:
    >> Can you elaborate? For kvm guests, the hardware is reasonably will implemented
    >> and if not we will fix it. We need not cripple a feature just because some
    >> hardware is broken.
    >>
    >
    > The short version is I don't know what work arounds we will ultimately
    > decide to deploy to work with real hardware.
    >
    > I have been seriously contemplating causing a cpu hot-unplug request
    > to fail if we are in ioapic mode and we have irqs routed to the cpu
    > that is being unplugged.
    >

    Well, obviously we need to disassociate any irqs from such a cpu. Could
    be done from the kernel or only enforced by the kernel.

    > Even with perfectly working hardware it is not possible in the general
    > case to migrate an ioapic irq from one cpu to another outside of an
    > interrupt handler without without risking dropping an interrupt.
    >

    Can't you generate a spurious interrupt immediately after the
    migration? An extra interrupt shouldn't hurt.

    > There is no general way to know you have seen the last interrupt
    > floating around your system. PCI ordering rules don't help because
    > the ioapics can potentially take an out of band channel.
    >

    Can you describe the problem scenario? an ioapic->lapic message
    delivered to a dead cpu?


    --
    I have a truly marvellous patch that fixes the bug which this
    signature is too narrow to contain.



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