From mboxrd@z Thu Jan  1 00:00:00 1970
From: Vitaly Kuznetsov <vkuznets@redhat.com>
Subject: Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V
Date: Wed, 14 Mar 2018 18:22:01 +0100
Message-ID: <87k1uebmiu.fsf@vitty.brq.redhat.com>
References: <20180309140249.2840-1-vkuznets@redhat.com>
        <20180309140249.2840-8-vkuznets@redhat.com>
        <alpine.DEB.2.21.1803091507300.1364@nanos.tec.linutronix.de>
        <87r2opcr6u.fsf@vitty.brq.redhat.com>
        <alpine.DEB.2.21.1803141618320.2481@nanos.tec.linutronix.de>
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Cc: Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        kvm@vger.kernel.org, x86@kernel.org,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@microsoft.com>,
        Mohammed Gamal <mmorsy@redhat.com>,
        Cathy Avery <cavery@redhat.com>, Bandan Das <bsd@redhat.com>,
        linux-kernel@vger.kernel.org
To: Thomas Gleixner <tglx@linutronix.de>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <alpine.DEB.2.21.1803141618320.2481@nanos.tec.linutronix.de>
        (Thomas Gleixner's message of "Wed, 14 Mar 2018 16:19:56 +0100 (CET)")
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

Thomas Gleixner <tglx@linutronix.de> writes:

> On Mon, 12 Mar 2018, Vitaly Kuznetsov wrote:
>> Thomas Gleixner <tglx@linutronix.de> writes:
>> > On Fri, 9 Mar 2018, Vitaly Kuznetsov wrote:
>> >> 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 for the tip,
>> 
>> with a single kernel user of these APIs it was easy to miss :-)
>> 
>> Unfortunately, these APIs are only present if HAVE_JUMP_LABEL and
>> (afaiu) we still care about KVM on !HAVE_JUMP_LABEL builds. It would be
>> nice if we can make them behave the same way static_branch_likely() and
>> friends do: compile into something else when !HAVE_JUMP_LABEL so we can
>> avoid nasty #ifdefs in C code.
>
> What's the reason for !jump label builds of a recent kernel? Old compilers?
>

To be honest I don't see any, we can start depending on HAVE_JUMP_LABEL
for CONFIG_KVM I guess.

-- 
  Vitaly