Re: [PATCH 0/1] x2apic implementation for kvm

[Avi Kivity <avi@redhat.com>]

Dong, Eddie wrote:
>> x2apic has the following benefit:
>>
>> - msr exits are faster than mmio (no page table walk, emulation)
>> - no need to read back ICR to look at the busy bit
>> - one ICR write instead of two
>> - potential to support large guests once we add interrupt remapping
>> - shared code with the Hyper-V paravirt interface
>>
>>     
> Is there any plan to implement an PV irqchip such as Xenirqchip for KVM?

No.  PV irqchips (and PV in general) have the following drawbacks:

- need to define and maintain an ABI
- only works on newer Linux guests
- obsoleted when hardware improves
- increase code size and maintenance effort
- have problems during transitions (boot, kexec)
- don't integrate well with device assignment
- require effort outside the kvm codebase

If a significant performance benefit can be demonstrated, I'll consider 
it, but until then my preference is full virtualization augmented by 
optional, targeted pv assists (like the TPR patching).

-- 
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