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    SubjectRe: [PATCH v3] enable x2APIC without interrupt remapping under KVM
    On Tue, Jun 30, 2009 at 02:24:05AM -0700, Eric W. Biederman wrote:
    > Gleb Natapov <gleb@redhat.com> writes:
    >
    > > KVM would like to provide x2APIC interface to a guest without emulating
    > > interrupt remapping device. The reason KVM prefers guest to use x2APIC
    > > is that x2APIC interface is better virtualizable and provides better
    > > performance than mmio xAPIC interface:
    > >
    > > - msr exits are faster than mmio (no page table walk, emulation)
    > > - no need to read back ICR to look at the busy bit
    > > - one 64 bit ICR write instead of two 32 bit writes
    > > - shared code with the Hyper-V paravirt interface
    > >
    > > Included patch changes x2APIC enabling logic to enable it even if IR
    > > initialization failed, but kernel runs under KVM and no apic id is
    > > greater than 255 (if there is one spec requires BIOS to move to x2apic
    > > mode before starting an OS).
    >
    >
    > How common is hotplug hardware in kvm? In particular hotplug cpus?
    >
    It works for Linux guests.

    > To support that seriously you need interrupt remapping.
    >
    Can you explain why?

    --
    Gleb.


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