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

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Message-ID: <87vah0w8h9.fsf@vitty.brq.redhat.com>
Date:   Thu, 21 Dec 2017 13:50:26 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, Bandan Das <bsd@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        linux-kernel@...r.kernel.org, devel@...uxdriverproject.org
Subject: Re: [PATCH RFC 0/7] KVM: nVMX: enlightened VMCS initial implementation

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> Paolo Bonzini <pbonzini@...hat.com> 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

-- 
  Vitaly