Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752375AbcC3JwG (ORCPT ); Wed, 30 Mar 2016 05:52:06 -0400 Received: from mail-wm0-f43.google.com ([74.125.82.43]:37375 "EHLO mail-wm0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750915AbcC3JwD (ORCPT ); Wed, 30 Mar 2016 05:52:03 -0400 Subject: Re: [PATCH v4 1/9] clocksource: arm_arch_timer: Gather KVM specific information in a structure To: Marc Zyngier , Christoffer Dall References: <1458842023-31853-1-git-send-email-julien.grall@arm.com> <1458842023-31853-2-git-send-email-julien.grall@arm.com> <56FAB7D2.1000508@linaro.org> <56FABC1F.2040500@arm.com> <20160330090621.GG4126@cbox> <56FB9881.9070209@arm.com> Cc: Julien Grall , kvmarm@lists.cs.columbia.edu, fu.wei@linaro.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, wei@redhat.com, al.stone@linaro.org, gg@slimlogic.co.uk, Thomas Gleixner From: Daniel Lezcano Message-ID: <56FBA1C0.7030108@linaro.org> Date: Wed, 30 Mar 2016 11:52:00 +0200 User-Agent: Mozilla/5.0 (X11; Linux i686; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 MIME-Version: 1.0 In-Reply-To: <56FB9881.9070209@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4741 Lines: 110 On 03/30/2016 11:12 AM, Marc Zyngier wrote: > On 30/03/16 10:06, Christoffer Dall wrote: >> On Tue, Mar 29, 2016 at 06:32:15PM +0100, Marc Zyngier wrote: >>> Daniel, >>> >>> On 29/03/16 18:13, Daniel Lezcano wrote: >>>> On 03/24/2016 06:53 PM, Julien Grall wrote: >>>>> Introduce a structure which are filled up by the arch timer driver and >>>>> used by the virtual timer in KVM. >>>>> >>>>> The first member of this structure will be the timecounter. More members >>>>> will be added later. >>>>> >>>>> A stub for the new helper isn't introduced because KVM requires the arch >>>>> timer for both ARM64 and ARM32. >>>>> >>>>> The function arch_timer_get_timecounter is kept for the time being and >>>>> will be dropped in a subsequent patch. >>>>> >>>>> Signed-off-by: Julien Grall >>>> >>>>> Cc: Daniel Lezcano >>>>> Cc: Thomas Gleixner >>>>> Cc: Marc Zyngier >>>>> >>>>> Changes in v3: >>>>> - Rename the patch >>>>> - Move the KVM changes and removal of arch_timer_get_timecounter >>>>> in separate patches. >>>>> --- >>>>> drivers/clocksource/arm_arch_timer.c | 12 +++++++++--- >>>>> include/clocksource/arm_arch_timer.h | 5 +++++ >>>>> 2 files changed, 14 insertions(+), 3 deletions(-) >>>>> >>>>> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c >>>>> index 5152b38..62bdfe7 100644 >>>>> --- a/drivers/clocksource/arm_arch_timer.c >>>>> +++ b/drivers/clocksource/arm_arch_timer.c >>>>> @@ -468,11 +468,16 @@ static struct cyclecounter cyclecounter = { >>>>> .mask = CLOCKSOURCE_MASK(56), >>>>> }; >>>>> >>>>> -static struct timecounter timecounter; >>>>> +static struct arch_timer_kvm_info arch_timer_kvm_info; >>>> >>>> This structure is statically defined in this subsystem but not used in >>>> this file and a couple of a accessors is added to let another subsystem >>>> to access it. >>>> >>>> That sounds there is something wrong here with the design of the current >>>> code, virt/phys are mixed. >>>> >>>> It isn't possible to split the virt/phys timer code respectively in >>>> virt/kvm/arm/arch_timer.c and drivers/clocksource/arm_arch_timer.c ? >>> >>> No, that'd be the wrong thing to do. The kernel uses *either* the virt >>> or phys timer depending on how it has been booted, and both counters are >>> in use. >>> >>> What KVM (or any other hypervisor) needs from the timer subsystem is: >>> - an interrupt (so that it can force a guest exit when the timer fires), >>> - a way to convert the values programmed into the HW into a timer event >>> (which is what the time counter structure is for). >>> >>> That allows the hypervisor to *emulate* a timer for the guest, and >>> that's what virt/kvm/arm/arch_timer.c is all about. We have a clear >>> separation of what is driving the HW vs what is emulating it, and I'm >>> quite eager to preserve that. >>> >>>> At least, 'struct arch_timer_kvm_info' should belong to >>>> virt/kvm/arm/arch_timer.c. >>> >>> At the cost of mandating separate storage in the arm_arch_timer driver. >>> I do not find that much nicer, but if you prefer that, fine by me. >>> >> If arch_timer_kvm_info is declared in virt/kvm/arm/arch_timer.c, then >> do you want to make it globally accessible and populated by this code or >> make it static to the KVM code and populate it with accessor functions? > > That'd be the latter, as I'm really not fond of global data. > >> To me the natural thing is that the arch timer driver maintains data >> about the device it drives, and consumers of that data can ask the arch >> timer driver for the details. > > That was my approach too, and that's the way the code proposed by Julien > works. Daniel seems to have a different take on it though. Well, I'm not against Julien's changes. The arm_arch_timer is complex and I don't have all the knowledge for the virt side. So I am just asking if everything is clearly separated which seems to be the case regarding your previous email. What sounds strange to me is we have a static global function which is not used (except at init time) by the timer and then we add accessors function to retrieve it. I would have expected arch_timer to pass a structure at init time to the timer driver and this one fills it. Then the arch timer can directly use its own structure. Anyway, perhaps I am splitting hairs. So up to you if you want to keep the current approach. -- Daniel -- Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog