Re: [PATCH RFC 0/7] KVM: nVMX: enlightened VMCS initial implementation

From: Vitaly Kuznetsov
Date: Thu Dec 21 2017 - 07:50:40 EST

Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> writes:

> Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
>> On 18/12/2017 18:17, Vitaly Kuznetsov wrote:
>>> The original author of these patches does no longer work at Red Hat, I
>>> agreed to take this over and send upstream. Here is his original
>>> description:
>>> "Makes KVM implement the enlightened VMCS feature per Hyper-V TLFS 5.0b.
>>> I've measured about %5 improvement in cost of a nested VM exit (Hyper-V
>>> enabled Windows Server 2016 nested in KVM)."
>> Can you try reproducing this and see how much a simple CPUID loop costs in:
>> * Hyper-V on Hyper-V (with enlightened VMCS, as a proxy for a full
>> implementation including the clean fields mask)
>> * Hyper-V on KVM, with and without enlightened VMCS
>> The latest kvm/queue branch already cut a lot of the cost of a nested VM
>> exit (from ~22000 to ~14000 clock cycles for KVM on KVM), so we could
>> also see if Hyper-V needs shadowing of more fields.
> I tested this series before sending out and was able to reproduce said
> 5% improvement with the feature (but didn't keep record of clock
> cycles). I'll try doing tests you mentioned on the same hardware and
> come back with the result. Hopefully I'll manage that before holidays.

I'm back with (somewhat frustrating) results (E5-2603):

1) Windows on Hyper-V (no nesting): 1350 cycles

2) Windows on Hyper-V on Hyper-V: 8600

3) Windows on KVM (no nesting): 1150 cycles

4) Windows on Hyper-V on KVM (no enlightened VMCS): 18200

5) Windows on Hyper-V on KVM (enlightened VMCS): 17100