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    SubjectRe: [PATCH v3] enable x2APIC without interrupt remapping under KVM
    On Tue, Jun 30, 2009 at 09:44:54AM -0700, Eric W. Biederman wrote:
    > Gleb Natapov <gleb@redhat.com> writes:
    >
    > > 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?
    >
    > Because ioapics don't fully function according to spec,
    > and the interrupt code on the hotplug path is a horrible
    > terrible broken hack for ioapics.
    >
    Considering that interrupt remapping is fairly new feature
    are you saying that hotplug (pci and cpu) on x86 is a horrible
    hack on Linux?

    > It is better than nothing but it certainly is not something
    > I would expect to work all of the time.
    >
    Because of horrible code or non-complaint ioapic implementation out
    there? If later this is not a big issue for KVM.

    > Interrupt remapping is the one case where we have hardware
    > that works according to spec and that works reasonably well.
    >
    I am sure when there was only one ioapic implementation in existence
    it worked according to a spec (and if not spec was changed :)) Give
    interrupt remapping some time and than we will see how above statement
    holds.

    --
    Gleb.


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