From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751244AbdAQRJR (ORCPT ); Tue, 17 Jan 2017 12:09:17 -0500 Received: from foss.arm.com ([217.140.101.70]:58516 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750941AbdAQRJQ (ORCPT ); Tue, 17 Jan 2017 12:09:16 -0500 Subject: Re: [RFC 0/8] Provide the EL1 physical timer to the VM To: Jintack Lim , kvmarm@lists.cs.columbia.edu, christoffer.dall@linaro.org References: <1482772326-29110-1-git-send-email-jintack@cs.columbia.edu> Cc: pbonzini@redhat.com, rkrcmar@redhat.com, linux@armlinux.org.uk, catalin.marinas@arm.com, will.deacon@arm.com, andre.przywara@arm.com, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org From: Marc Zyngier Organization: ARM Ltd Message-ID: Date: Tue, 17 Jan 2017 17:09:12 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Icedove/45.5.1 MIME-Version: 1.0 In-Reply-To: <1482772326-29110-1-git-send-email-jintack@cs.columbia.edu> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 26/12/16 17:11, Jintack Lim wrote: > The ARM architecture defines the EL1 physical timer and the virtual > timer, and it is reasonable for an OS to expect to be able to access > both. However, the current KVM implementation does not provide the EL1 > physical timer to VMs but terminates VMs on access to the timer. > > On VHE systems, this would be as simple as allowing full access to the > EL1 physical timer to VMs because the KVM host does not use the EL1 > physical timer. However, on non-VHE systems, the KVM host already uses > the EL1 physical timer which prevents us from granting full access of > the EL1 physical timer to VMs. > > This patchset enables VMs to use the EL1 physical timer through > trap-and-emulate. The KVM host emulates each EL1 physical timer > register access and sets up the background timer accordingly. When the > background timer expires, the KVM host injects EL1 physical timer > interrupts to the VM. Alternatively, it's also possible to allow VMs to > access the EL1 physical timer without trapping. However, this requires > somehow using the EL2 physical timer for the Linux host while running > the VM instead of the EL1 physical timer. Right now I just implemented > trap-and-emulate because this was straightforward to do, and I leave it > to future work to determine if transferring the EL1 physical timer state > to the EL2 timer provides any performance benefit. > > This feature will be useful for any OS that wishes to access the EL1 > physical timer. Nested virtualization is one of those use cases. A > nested hypervisor running inside a VM would think it has full access to > the hardware and naturally tries to use the EL1 physical timer as Linux > would do. Other nested hypervisors may try to use the EL2 physical timer > as Xen would do, but supporting the EL2 physical timer to the VM is out > of scope of this patchset. This patchset will make it easy to add the > EL2 timer support in the future, though. > > Note, Linux VMs booting in EL1 will be unaffected by this patch set and > will continue to use only the virtual timer and this patch set will > therefore not introduce any performance degredation as a result of > trap-and-emulate. Hi Jintack, Any chance you could address Christoffer's comments and respin this series? This looks like a good enhancement to our emulation, and definitely a requirement for the nested work, so I'm obviously keen on it. Thanks, M. -- Jazz is not dead. It just smells funny...