From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751342AbdBXMRS (ORCPT ); Fri, 24 Feb 2017 07:17:18 -0500 Received: from mx1.redhat.com ([209.132.183.28]:55906 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751082AbdBXMRK (ORCPT ); Fri, 24 Feb 2017 07:17:10 -0500 Subject: Re: [patch 0/3] KVM CPU frequency change hypercalls To: Marcelo Tosatti References: <20170202174755.946578704@redhat.com> <20170203164349.GA5582@potion> <20170203181405.GA1869@amt.cnet> <20170203190930.GD15128@potion> <5db0e1a1-2eaf-fe77-3e15-7b2ef842d255@redhat.com> <20170223231923.GB23631@amt.cnet> <5310a7a1-3c3b-7b19-4f2d-6f7919c7b560@redhat.com> <20170224114958.GA28618@amt.cnet> Cc: Radim Krcmar , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, "Rafael J. Wysocki" , Viresh Kumar From: Paolo Bonzini Message-ID: <70575c62-b9c5-7100-193a-5f4b8bed258c@redhat.com> Date: Fri, 24 Feb 2017 13:17:07 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0 MIME-Version: 1.0 In-Reply-To: <20170224114958.GA28618@amt.cnet> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Fri, 24 Feb 2017 12:17:10 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 24/02/2017 12:50, Marcelo Tosatti wrote: >>> >>> On all other cpufreq implementations, these boundaries still need to >>> be set. Then, a "governor" must be selected. Such a "governor" decides >>> what speed the processor shall run within the boundaries. One such >>> "governor" is the "userspace" governor. This one allows the user - or >>> a yet-to-implement userspace program - to decide what specific speed >>> the processor shall run at. >> The userspace program sets a policy for the whole system. > No, its per cpu. Yeah, what I mean is that userspace program can be per-CPU, but it looks at all the processes running on that CPU ("the whole system"). This is very different from a guest, which is isolated. >>>> That's bad. This feature is broken by design unless it does proper >>>> save/restore across preemption. >>> Whats the current usecase, or forseeable future usecase, for save/restore >>> across preemption again? (which would validate the broken by design >>> claim). >> Stop a guest that is using cpufreq, start a guest that is not using it. >> The second guest's performance now depends on the state that the first >> guest left in cpufreq. > > Nothing forbids the host to implement switching with the > current hypercall interface: all you need is a scheduler > hook. Can it be done in vcpu_load/vcpu_put? But you still would have two components (KVM and sysfs) potentially fighting over the frequency, and that's still a bit ugly. Paolo