From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752866AbaIKRWx (ORCPT ); Thu, 11 Sep 2014 13:22:53 -0400 Received: from service87.mimecast.com ([91.220.42.44]:45497 "EHLO service87.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752705AbaIKRWw convert rfc822-to-8bit (ORCPT ); Thu, 11 Sep 2014 13:22:52 -0400 Message-ID: <5411DA67.2040402@arm.com> Date: Thu, 11 Sep 2014 18:22:47 +0100 From: Marc Zyngier User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130330 Thunderbird/17.0.5 MIME-Version: 1.0 To: Doug Anderson CC: Will Deacon , "olof@lixom.net" , Sonny Rao , Catalin Marinas , Mark Rutland , Stephen Boyd , Sudeep Holla , Christopher Covington , Lorenzo Pieralisi , Thomas Gleixner , Daniel Lezcano , Nathan Lynch , "linux-arm-kernel@lists.infradead.org" , "robh+dt@kernel.org" , Pawel Moll , "ijc+devicetree@hellion.org.uk" , "galak@codeaurora.org" , "devicetree@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v2] clocksource: arch_timer: Allow the device tree to specify the physical timer References: <1410452204-7277-1-git-send-email-dianders@chromium.org> <20140911164710.GW6158@arm.com> <5411D528.4050605@arm.com> In-Reply-To: X-Enigmail-Version: 1.4.6 X-OriginalArrivalTime: 11 Sep 2014 17:22:48.0587 (UTC) FILETIME=[023E35B0:01CFCDE5] X-MC-Unique: 114091118224906401 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/09/14 18:11, Doug Anderson wrote: > Hi, > > On Thu, Sep 11, 2014 at 10:00 AM, Marc Zyngier wrote: >> On 11/09/14 17:47, Will Deacon wrote: >>> On Thu, Sep 11, 2014 at 05:16:44PM +0100, Doug Anderson wrote: >>>> Some 32-bit (ARMv7) systems are architected like this: >>>> >>>> * The firmware doesn't know and doesn't care about hypervisor mode and >>>> we don't want to add the complexity of hypervisor there. >>>> >>>> * The firmware isn't involved in SMP bringup or resume. >>>> >>>> * The ARCH timer come up with an uninitialized offset between the >>>> virtual and physical counters. Each core gets a different random >>>> offset. >>>> >>>> On systems like the above, it doesn't make sense to use the virtual >>>> counter. There's nobody managing the offset and each time a core goes >>>> down and comes back up it will get reinitialized to some other random >>>> value. >>> >>> You probably need to rephrase this slightly, as there *is* still a >>> requirement on the hypervisor/firmware (actually, two!). See below. >>> >>>> Let's add a property to the device tree to say that we shouldn't use >>>> the virtual timer. Firmware could potentially remove this property >>>> before passing the device tree to the kernel if it really wants the >>>> kernel to use a virtual timer. >>>> >>>> Note that it's been said that ARM64 (ARMv8) systems the firmware and >>>> kernel really can't be architected as described above. That means >>>> using the physical timer like this really only makes sense for ARMv7 >>>> systems. >>> >>> I'd go further: this only makes sense if you're booting in secure SVC >>> mode. >> >> If that's the case, what's the problem? Enter monitor mode, set SCR.NS >> to one, nuke CNTVOFF, revert, job done. >> >> What am I missing? > > Stuff like this was talked about in the thread about Sonny's patch at > > > ...in that case we were always talking about HYP mode, though. I That's because I always assumed that you'd be running non-secure, dropped there by some idiotic firmware without any way to go back up. > don't think anyone has explicitly talked about just switching to > monitor mode and then leaving ourselves in Secure SVC after we're > done. It would be nice (especially for the VDSO guys) if we could > just init the virtual offset... > > We would need to run this code potentially at processor bringup and > after suspend/resume, but that seems possible too. Note that this would be an ARMv7 only thing (you can't do that on ARMv8, at all). > Is the transition to monitor mode and back simple? Where would you > suggest putting this code? It would definitely need to be pretty > early. We'd also need to be able to detect that we're in Secure SVC > and not mess up anyone else who happened to boot in Non Secure SVC. This would have to live in some very early platform-specific code. The ugly part is that you cannot find out what world you're in (accessing SCR is going to send you to UNDEF-land if accessed from NS). If I was suicidal, I'd suggest you could pass a parameter to the command line, interpreted by the timer code... But I since I'm not, let's pretend I haven't said anything... ;-) M. -- Jazz is not dead. It just smells funny...