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 15:54:57 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     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

On 12/03/2018 15:19, 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.
> 
> That said I'd like to defer the question to KVM maintainers: Paolo,
> Radim, what would you like me to do? Use STATIC_JUMP_IF_TRUE/FALSE as
> they are, try to make them work for !HAVE_JUMP_LABEL and use them or
> maybe we can commit the series as-is and have it as a future
> optimization (e.g. when HAVE_JUMP_LABEL becomes mandatory)?

With a single instruction to patch, poking at the text manually might be
an option...  Otherwise, it's okay as-is.

Paolo