From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [RFC PATCH] KVM: Only register preempt notifiers and load arch cpu state as needed Date: Thu, 23 Nov 2017 18:12:41 +0100 Message-ID: <62ae4eb1-fd57-c525-cd73-e3f646d340e1@redhat.com> References: <20171123160521.27260-1-christoffer.dall@linaro.org> <72357599-798d-14d0-336a-69a083f17863@redhat.com> <20171123170642.GA28855@cbox> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-mips@linux-mips.org, kvm@vger.kernel.org, Marc Zyngier , James Hogan , Cornelia Huck , kvm-ppc@vger.kernel.org, Christian Borntraeger , linux-arm-kernel@lists.infradead.org, Alexander Graf , linux-s390@vger.kernel.org, kvmarm@lists.cs.columbia.edu To: Christoffer Dall Return-path: In-Reply-To: <20171123170642.GA28855@cbox> Content-Language: en-US List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu List-Id: kvm.vger.kernel.org On 23/11/2017 18:06, Christoffer Dall wrote: > On Thu, Nov 23, 2017 at 05:17:00PM +0100, Paolo Bonzini wrote: >> On 23/11/2017 17:05, Christoffer Dall wrote: >>> For example, >>> arm64 is about to do significant work in vcpu load/put when running a >>> vcpu, but not when doing things like KVM_SET_ONE_REG or >>> KVM_SET_MP_STATE. >> >> Out of curiosity, in what circumstances are these ioctls a hot path? >> Especially KVM_SET_MP_STATE. > > Perhaps my commit message was misleading; we only want to do that for > KVM_RUN, and not for anything else. We're already doing things like > potentially jumping to hyp mode and flushing VMIDs which really > shouldn't be done unless we actually plan on running a VCPU, and we're > going to do things like setting up the timer to handle timer interrupts > in an ISR, which doesn't make sense unless the VCPU is running. > > Add to that, that loading an entire VM's state onto hardware, only to > read back a single register from hardware and returning it to user > space, doesn't really fall within optimization vs. non-optimization in > the critical path, but is just wrong, IMHO. For GET/SET_ONE_REG it certainly makes sense. For everything else, I'm wondering which ioctls (and how many calls to each of them) exactly you are seeing, and also on which userspace paths. > That doesn't solve my need as I want to *only* do the arch vcpu_load for > KVM_RUN, I should have been more clear in the commit message. That's what you want to do, but it might not be what you need to do. Paolo