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

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

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.

It is better than nothing but it certainly is not something
I would expect to work all of the time.

Interrupt remapping is the one case where we have hardware
that works according to spec and that works reasonably well.

Eric