From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751780AbeCLOTM (ORCPT ); Mon, 12 Mar 2018 10:19:12 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:44748 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751232AbeCLOTL (ORCPT ); Mon, 12 Mar 2018 10:19:11 -0400 From: Vitaly Kuznetsov To: Paolo Bonzini , Radim =?utf-8?B?S3LEjW3DocWZ?= , Thomas Gleixner Cc: kvm@vger.kernel.org, x86@kernel.org, "K. Y. Srinivasan" , Haiyang Zhang , Stephen Hemminger , "Michael Kelley \(EOSG\)" , Mohammed Gamal , Cathy Avery , Bandan Das , linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V References: <20180309140249.2840-1-vkuznets@redhat.com> <20180309140249.2840-8-vkuznets@redhat.com> Date: Mon, 12 Mar 2018 15:19:05 +0100 In-Reply-To: (Thomas Gleixner's message of "Fri, 9 Mar 2018 15:08:34 +0100 (CET)") Message-ID: <87r2opcr6u.fsf@vitty.brq.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Thomas Gleixner writes: > On Fri, 9 Mar 2018, Vitaly Kuznetsov wrote: > >> Enlightened VMCS is just a structure in memory, the main benefit >> besides avoiding somewhat slower VMREAD/VMWRITE is using clean field >> mask: we tell the underlying hypervisor which fields were modified >> since VMEXIT so there's no need to inspect them all. >> >> Tight CPUID loop test shows significant speedup: >> Before: 18890 cycles >> After: 8304 cycles >> >> 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)? -- Vitaly