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

From: Thomas Gleixner <tglx@linutronix.de>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: "Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <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
Subject: Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V
Date: Wed, 14 Mar 2018 16:19:56 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.21.1803141618320.2481@nanos.tec.linutronix.de> (raw)
In-Reply-To: <87r2opcr6u.fsf@vitty.brq.redhat.com>

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?

Thanks,

	tglx