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Tue, 12 Nov 2019 09:42:43 +0000 (UTC) Received: from paraplu.localdomain (unknown [10.36.118.15]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 4426962671; Tue, 12 Nov 2019 09:42:40 +0000 (UTC) Received: by paraplu.localdomain (Postfix, from userid 1001) id 70BFE3E044D; Tue, 12 Nov 2019 10:42:38 +0100 (CET) Date: Tue, 12 Nov 2019 10:42:38 +0100 From: Kashyap Chamarthy To: qemu-devel@nongnu.org Subject: Re: [PATCH qemu-web] Add a blog post on "Micro-Optimizing KVM VM-Exits" Message-ID: <20191112094238.GK7754@paraplu> References: <20191108092247.16207-1-kchamart@redhat.com> MIME-Version: 1.0 In-Reply-To: <20191108092247.16207-1-kchamart@redhat.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-MC-Unique: 2Oy9Gp9CORGbvWME0WjbYQ-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline 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, dgilbert@redhat.com, rjones@redhat.com, stefanha@redhat.com, pbonzini@redhat.com, vkuznets@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" [Cc: Rich Jones, addressing his feedback on IRC, below.] On Fri, Nov 08, 2019 at 10:22:47AM +0100, 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 > [1] https://kvmforum2019.sched.com/event/Tmwr/micro-optimizing-kvm-vm-exi= ts-andrea-arcangeli-red-hat-inc >=20 > Signed-off-by: Kashyap Chamarthy > --- [...] > +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. I can send a v2 (but will wait for any other feedback), or when applying someone please replace the above paragraph with the following: "Andrea constructed a synthetic microbenchmark program (without any GCC optimizations or caching) which runs the CPUID instructions one million times in a loop. This microbenchmark is meant to focus on measuring the performance of a specific area of the code -- in this case, to test the latency of VM-Exits." (Rich, hope that reads better. Thanks for the review.) > +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. [...] --=20 /kashyap