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Date:   Wed, 14 Mar 2018 20:59:25 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
cc:     Paolo Bonzini <pbonzini@redhat.com>,
        =?ISO-8859-2?Q?Radim_Kr=E8m=E1=F8?= <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>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on
 Hyper-V
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On Wed, 14 Mar 2018, Vitaly Kuznetsov wrote:
> 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.

We currently try to move the minimum compiler version to one which provides
jump label support, so this should be a non issue.

@Peter: What was the final conclusion of this discussion?

Thanks,

	tglx