linux-kernel - Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V

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Message-ID: <alpine.DEB.2.21.1803091507300.1364@nanos.tec.linutronix.de>
Date:   Fri, 9 Mar 2018 15:08:34 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
cc:     kvm@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        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>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on
 Hyper-V

On Fri, 9 Mar 2018, Vitaly Kuznetsov wrote:

> Enlightened VMCS is just a structure in memory, the main benefit
> besides avoiding somewhat slower VMREAD/VMWRITE is using clean field
> mask: we tell the underlying hypervisor which fields were modified
> since VMEXIT so there's no need to inspect them all.
> 
> Tight CPUID loop test shows significant speedup:
> Before: 18890 cycles
> After: 8304 cycles
> 
> Static key is being used to avoid performance penalty for non-Hyper-V
> deployments. Tests show we add around 3 (three) CPU cycles on each
> VMEXIT (1077.5 cycles before, 1080.7 cycles after for the same CPUID
> loop on bare metal). We can probably avoid one test/jmp in vmx_vcpu_run()
> but I don't see a clean way to use static key in assembly.

STATIC_JUMP_IF_TRUE, STATIC_JUMP_IF_FALSE are your friends.

Thanks,

	tglx