From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DD5FFC432C3 for ; Fri, 15 Nov 2019 12:13:34 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id ABC1420748 for ; Fri, 15 Nov 2019 12:13:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="dtg2tckZ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org ABC1420748 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:38178 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iVaTl-0005c3-Oq for qemu-devel@archiver.kernel.org; Fri, 15 Nov 2019 07:13:33 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:41959) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iVaPU-0000jA-Ir for qemu-devel@nongnu.org; Fri, 15 Nov 2019 07:09:10 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iVaPS-0007bb-5F for qemu-devel@nongnu.org; Fri, 15 Nov 2019 07:09:07 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:38573 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iVaPS-0007ab-0i for qemu-devel@nongnu.org; Fri, 15 Nov 2019 07:09:06 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1573819742; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=FyrX4rcI1HWv0Cg5BafeF4SUcaqWoHOIwp1NeoMFa9g=; b=dtg2tckZYgCVGtMRj3dKGZs5zgR8zGg0oK/g4RU2PHK3be0ftlqa0PAvifsAbfoKARBRp1 ZwuzC3BfwRyDeBJ4Ik0RLcFHD1A0TjUTkkiob/yTWuVToZBSS5CSXPkB11losqrZ3THQDQ 9Id2yKH2HldtTZBm1S5fKN1f2iTKQRA= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-228-DyMqPIisPCKlCPThZ6ZjeQ-1; Fri, 15 Nov 2019 07:09:00 -0500 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C297C8024E5 for ; Fri, 15 Nov 2019 12:08:59 +0000 (UTC) Received: from localhost.localdomain (ovpn-117-14.ams2.redhat.com [10.36.117.14]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 2257BF6DE; Fri, 15 Nov 2019 12:08:54 +0000 (UTC) Subject: Re: [PATCH qemu-web] Add a blog post on "Micro-Optimizing KVM VM-Exits" To: Kashyap Chamarthy , qemu-devel@nongnu.org References: <20191108092247.16207-1-kchamart@redhat.com> From: Thomas Huth Message-ID: Date: Fri, 15 Nov 2019 13:08:53 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.0 MIME-Version: 1.0 In-Reply-To: <20191108092247.16207-1-kchamart@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-MC-Unique: DyMqPIisPCKlCPThZ6ZjeQ-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.120 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: aarcange@redhat.com, pbonzini@redhat.com, vkuznets@redhat.com, dgilbert@redhat.com, stefanha@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 08/11/2019 10.22, Kashyap Chamarthy wrote: > This blog post summarizes the talk "Micro-Optimizing KVM VM-Exits"[1], > given by Andrea Arcangeli at the recently concluded KVM Forum 2019. >=20 Hi Kashyap, first thanks for writing up this article! It's a really nice summary of the presentation, I think. But before we include it, let me ask a meta-question: Is an article about the KVM *kernel* code suitable for the *QEMU* blog? Or is there maybe a better place for this, like an article on www.linux-kvm.org ? Opinions? Ideas? Thomas > --- > ...019-11-06-micro-optimizing-kvm-vmexits.txt | 115 ++++++++++++++++++ > 1 file changed, 115 insertions(+) > create mode 100644 _posts/2019-11-06-micro-optimizing-kvm-vmexits.txt >=20 > diff --git a/_posts/2019-11-06-micro-optimizing-kvm-vmexits.txt b/_posts/= 2019-11-06-micro-optimizing-kvm-vmexits.txt > new file mode 100644 > index 0000000000000000000000000000000000000000..f4a28d58ddb40103dd599fdfd= 861eeb4c41ed976 > --- /dev/null > +++ b/_posts/2019-11-06-micro-optimizing-kvm-vmexits.txt > @@ -0,0 +1,115 @@ > +--- > +layout: post > +title: "Micro-Optimizing KVM VM-Exits" > +date: 2019-11-08 > +categories: [kvm, optimization] > +--- > + > +Background on VM-Exits > +---------------------- > + > +KVM (Kernel-based Virtual Machine) is the Linux kernel module that > +allows a host to run virtualized guests (Linux, Windows, etc). The KVM > +"guest execution loop", with QEMU (the open source emulator and > +virtualizer) as its user space, is roughly as follows: QEMU issues the > +ioctl(), KVM_RUN, to tell KVM to prepare to enter the CPU's "Guest Mode" > +-- a special processor mode which allows guest code to safely run > +directly on the physical CPU. The guest code, which is inside a "jail" > +and thus cannot interfere with the rest of the system, keeps running on > +the hardware until it encounters a request it cannot handle. Then the > +processor gives the control back (referred to as "VM-Exit") either to > +kernel space, or to the user space to handle the request. Once the > +request is handled, native execution of guest code on the processor > +resumes again. And the loop goes on. > + > +There are dozens of reasons for VM-Exits (Intel's Software Developer > +Manual outlines 64 "Basic Exit Reasons"). For example, when a guest > +needs to emulate the CPUID instruction, it causes a "light-weight exit" > +to kernel space, because CPUID (among a few others) is emulated in the > +kernel itself, for performance reasons. But when the kernel _cannot_ > +handle a request, e.g. to emulate certain hardware, it results in a > +"heavy-weight exit" to QEMU, to perform the emulation. These VM-Exits > +and subsequent re-entries ("VM-Enters"), even the light-weight ones, can > +be expensive. What can be done about it? > + > +Guest workloads that are hard to virtualize > +------------------------------------------- > + > +At the 2019 edition of the KVM Forum in Lyon, kernel developer, Andrea > +Arcangeli, attempted to address the kernel part of minimizing VM-Exits. > + > +His talk touched on the cost of VM-Exits into the kernel, especially for > +guest workloads (e.g. enterprise databases) that are sensitive to their > +performance penalty. However, these workloads cannot avoid triggering > +VM-Exits with a high frequency. Andrea then outlined some of the > +optimizations he's been working on to improve the VM-Exit performance in > +the KVM code path -- especially in light of applying mitigations for > +speculative execution flaws (Spectre v2, MDS, L1TF). > + > +Andrea gave a brief recap of the different kinds of speculative > +execution attacks (retpolines, IBPB, PTI, SSBD, etc). Followed by that > +he outlined the performance impact of Spectre-v2 mitigations in context > +of KVM. > + > +The microbechmark: CPUID in a one million loop > +---------------------------------------------- > + > +The synthetic microbenchmark (meaning, focus on measuring the > +performance of a specific area of code) Andrea used was to run the CPUID > +instruction one million times, without any GCC optimizations or caching. > +This was done to test the latency of VM-Exits. > + > +While stressing that the results of these microbenchmarks do not > +represent real-world workloads, he had two goals in mind with it: (a) > +explain how the software mitigation works; and (b) to justify to the > +broader community the value of the software optimizations he's working > +on in KVM. > + > +Andrea then reasoned through several interesting graphs that show how > +CPU computation time gets impacted when you disable or enable the > +various kernel-space mitigations for Spectre v2, L1TF, MDS, et al. > + > +The proposal: "KVM Monolithic" > +------------------------------ > + > +Based on his investigation, Andrea proposed a patch series, ["KVM > +monolithc"](https://lwn.net/Articles/800870/), to get rid of the KVM > +common module, 'kvm.ko'. Instead the KVM common code gets linked twice > +into each of the vendor-specific KVM modules, 'kvm-intel.ko' and > +'kvm-amd.ko'. > + > +The reason for doing this is that the 'kvm.ko' module indirectly calls > +(via the "retpoline" technique) the vendor-specific KVM modules at every > +VM-Exit, several times. These indirect calls were not optimal before, > +but the "retpoline" mitigation (which isolates indirect branches, that > +allow a CPU to execute code from arbitrary locations, from speculative > +execution) for Spectre v2 compounds the problem, as it degrades > +performance. > + > +This approach will result in a few MiB of increased disk space for > +'kvm-intel.ko' and 'kvm-amd.ko', but the upside in saved indirect calls, > +and the elimination of "retpoline" overhead at run-time more than > +compensate for it. > + > +With the "KVM Monolithic" patch series applied, Andrea's microbenchmarks > +show a double-digit improvement in performance with default mitigations > +(for Spectre v2, et al) enabled on both Intel 'VMX' and AMD 'SVM'. And > +with 'spectre_v2=3Doff' or for CPUs with IBRS_ALL in ARCH_CAPABILITIES > +"KVM monolithic" still improve[s] performance, albiet it's on the order > +of 1%. > + > +Conclusion > +---------- > + > +Removal of the common KVM module has a non-negligible positive > +performance impact. And the "KVM Monolitic" patch series is still > +actively being reviewed, modulo some pending clean-ups. Based on the > +upstream review discussion, KVM Maintainer, Paolo Bonzini, and other > +reviewers seemed amenable to merge the series. > + > +Although, we still have to deal with mitigations for 'indirect branch > +prediction' for a long time, reducing the VM-Exit latency is important > +in general; and more specifically, for guest workloads that happen to > +trigger frequent VM-Exits, without having to disable Spectre v2 > +mitigations on the host, as Andrea stated in the cover letter of his > +patch series. >=20