From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [patch 0/3] KVM CPU frequency change hypercalls Date: Thu, 23 Feb 2017 18:35:24 +0100 Message-ID: <5db0e1a1-2eaf-fe77-3e15-7b2ef842d255@redhat.com> References: <20170202174755.946578704@redhat.com> <20170203164349.GA5582@potion> <20170203181405.GA1869@amt.cnet> <20170203190930.GD15128@potion> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, "Rafael J. Wysocki" , Viresh Kumar To: Radim Krcmar , Marcelo Tosatti Return-path: In-Reply-To: <20170203190930.GD15128@potion> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 03/02/2017 20:09, Radim Krcmar wrote: > One reason why we have a kernel/userspace split is to allow sharing of > CPU time. Each application then its state that the kernel keeps track > of and saves/restores while time-multiplexing. > > Our frequency scaling interface goes against the idea -- guest kernel > cannot schedule multiple userspaces on the same vCPU, because they could > conflict by overriding frequency. > > i.e. our feature implies userspace tasks pinned to isolated vCPUs. That's bad. This feature is broken by design unless it does proper save/restore across preemption. You don't need a hypercall. Add a cpufreq driver in DPDK that doesn't use sysfs, and connect it to a daemon in the host through virtio-serial or vsock. Paolo