Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM

Gleb Natapov
2009-06-30T09:26:22Z

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.