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

[ebiederm@xmission.com (Eric W. Biederman)]

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?

To support that seriously you need interrupt remapping.

Eric