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

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Date:   Wed, 14 Mar 2018 18:22:01 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, x86@...nel.org,
        "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

Thomas Gleixner <tglx@...utronix.de> writes:

> On Mon, 12 Mar 2018, Vitaly Kuznetsov wrote:
>> Thomas Gleixner <tglx@...utronix.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