Re: QEMU PIC indirection patch for in-kernel APIC work

Avi Kivity
2007-04-05T13:59:17Z

Dong, Eddie wrote:
> Dor Laor wrote:
>
>> Do you know why Xen guy choose of implementing it in Xen? Why didn't
>> they rip Qemu implementation?
>>
>>
> With PIC in Xen, CPU2K gets 6.5% performance gain in old 1000HZ linux
> kernel, KB gets 14% gain. We also did a shared PIC model which share
> PIC state among Qemu & VMM with less LOC in VMM, it can get
> similar performance gain (5.8% in my test).
> BTW, at that time, PIT is in VMM already.
>

I expect that the gain in kvm will be smaller. Xen has to schedule dom0
to process the event channel (possibly on another cpu), dom0 has to
schedule qemu-dm (again, possibly on another cpu), qemu does its thing,
and then Xen has to schedule domU again. With kvm, we are always on the
same cpu, and the only overhead is the system call, which is a few
hundred nanoseconds. I expect with current hardware that it will be
negligible (as a vmexit is measured in microseconds), but to become
measurable as hardware improves.

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