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    SubjectRe: [GIT PULL] Xen APIC hooks (with io_apic_ops)

    * Avi Kivity <> wrote:

    > Ingo Molnar wrote:
    >>> IO APIC operations are not even slightly performance critical? Are
    >>> they ever used on the interrupt delivery path?
    >> Since they are not performance critical, then why doesnt Xen catch the
    >> IO-APIC accesses, and virtualizes the device?
    >> If you want to hook into the IO-APIC code at such a low level, why
    >> dont you hook into the _hardware_ API - i.e. catch those setup/routing
    >> modifications to the IO-APIC space. No Linux changes are needed in that
    >> case.
    > When x2apic is enabled, and EOI broadcast is disabled, then the io
    > apic does become a hot path - it needs to be written for each
    > level-triggered interrupt EOI. In this case I might want to
    > paravirtualize the EOI write to exit only if an interrupt is
    > pending; otherwise communicate via shared memory.
    > We do something similar for Windows (by patching it) very
    > successfully; Windows likes to touch the APIC TPR ~ 100,000 times
    > per second, usually without triggering an interrupt. We hijack
    > these writes, do the checks in guest context, and only exit if the
    > TPR write would trigger an interrupt.

    I suspect you aware of that this is about the io-apic not the local
    APIC. The local apic methods are already driver-ized - and they sit
    closer to the CPU so they matter more to performance.

    > (kvm will likely gain x2apic support in 2.6.32; patches have
    > already been posted)

    ok. This points in the direction of the io-apic driver abstraction
    from Jeremy being the right long-term approach. We already have a
    few quirks that could be cleaned up by using a proper driver


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      Last update: 2009-05-25 05:55    [W:0.022 / U:22.724 seconds]
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