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Date: Fri, 9 Mar 2018 15:08:34 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
cc: kvm@vger.kernel.org, x86@kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
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        Mohammed Gamal <mmorsy@redhat.com>, Cathy Avery <cavery@redhat.com>,
        Bandan Das <bsd@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on
 Hyper-V
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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