Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966002AbbDVPkr (ORCPT ); Wed, 22 Apr 2015 11:40:47 -0400 Received: from mail-wg0-f52.google.com ([74.125.82.52]:34845 "EHLO mail-wg0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S964995AbbDVPkj (ORCPT ); Wed, 22 Apr 2015 11:40:39 -0400 Message-ID: <5537C0F4.7030702@linaro.org> Date: Wed, 22 Apr 2015 16:40:36 +0100 From: Daniel Thompson User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: Marc Zyngier CC: Thomas Gleixner , Jason Cooper , Russell King , Will Deacon , Catalin Marinas , Stephen Boyd , John Stultz , Steven Rostedt , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "patches@linaro.org" , "linaro-kernel@lists.linaro.org" , Sumit Semwal , Dirk Behme , Daniel Drake , Dmitry Pervushin , Tim Sander Subject: Re: [RESEND PATCH 4.0-rc7 v20 3/6] irqchip: gic: Introduce plumbing for IPI FIQ References: <1427216014-5324-1-git-send-email-daniel.thompson@linaro.org> <1428659511-9590-1-git-send-email-daniel.thompson@linaro.org> <1428659511-9590-4-git-send-email-daniel.thompson@linaro.org> <55365477.30404@arm.com> <5536BB1D.6040908@linaro.org> <20150422101521.606aa38f@arm.com> <553797ED.8040504@linaro.org> <20150422135729.2a0beb86@why.wild-wind.fr.eu.org> In-Reply-To: <20150422135729.2a0beb86@why.wild-wind.fr.eu.org> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7037 Lines: 153 On 22/04/15 13:57, Marc Zyngier wrote: > On Wed, 22 Apr 2015 13:45:33 +0100 > Daniel Thompson wrote: > >> On 22/04/15 10:15, Marc Zyngier wrote: >>> On Tue, 21 Apr 2015 22:03:25 +0100 >>> Daniel Thompson wrote: >>> >>> Hi Daniel, >>> >>>> On 21/04/15 14:45, Marc Zyngier wrote: >>>>> On 10/04/15 10:51, Daniel Thompson wrote: >>>>>> Currently it is not possible to exploit FIQ for systems with a GIC, even if >>>>>> the systems are otherwise capable of it. This patch makes it possible >>>>>> for IPIs to be delivered using FIQ. >>>>>> >>>>>> To do so it modifies the register state so that normal interrupts are >>>>>> placed in group 1 and specific IPIs are placed into group 0. It also >>>>>> configures the controller to raise group 0 interrupts using the FIQ >>>>>> signal. It provides a means for architecture code to define which IPIs >>>>>> shall use FIQ and to acknowledge any IPIs that are raised. >>>>>> >>>>>> All GIC hardware except GICv1-without-TrustZone support provides a means >>>>>> to group exceptions into group 0 and group 1 but the hardware >>>>>> functionality is unavailable to the kernel when a secure monitor is >>>>>> present because access to the grouping registers are prohibited outside >>>>>> "secure world". However when grouping is not available (or in the case >>>>>> of early GICv1 implementations is very hard to configure) the code to >>>>>> change groups does not deploy and all IPIs will be raised via IRQ. >>>>>> >>>>>> It has been tested and shown working on two systems capable of >>>>>> supporting grouping (Freescale i.MX6 and STiH416). It has also been >>>>>> tested for boot regressions on two systems that do not support grouping >>>>>> (vexpress-a9 and Qualcomm Snapdragon 600). >>>>>> >>>>>> Signed-off-by: Daniel Thompson >>>>>> Cc: Thomas Gleixner >>>>>> Cc: Jason Cooper >>>>>> Cc: Russell King >>>>>> Cc: Marc Zyngier >>>>>> Tested-by: Jon Medhurst >>>>>> --- >>>>>> arch/arm/kernel/traps.c | 5 +- >>>>>> drivers/irqchip/irq-gic.c | 151 +++++++++++++++++++++++++++++++++++++--- >>>>>> include/linux/irqchip/arm-gic.h | 8 +++ >>>>>> 3 files changed, 153 insertions(+), 11 deletions(-) >>>>>> >>>>>> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c >>>>>> index 788e23fe64d8..b35e220ae1b1 100644 >>>>>> --- a/arch/arm/kernel/traps.c >>>>>> +++ b/arch/arm/kernel/traps.c >>>>>> @@ -26,6 +26,7 @@ >>>>>> #include >>>>>> #include >>>>>> #include >>>>>> +#include >>>>>> >>>>>> #include >>>>>> #include >>>>>> @@ -479,7 +480,9 @@ asmlinkage void __exception_irq_entry handle_fiq_as_nmi(struct pt_regs *regs) >>>>>> >>>>>> nmi_enter(); >>>>>> >>>>>> - /* nop. FIQ handlers for special arch/arm features can be added here. */ >>>>>> +#ifdef CONFIG_ARM_GIC >>>>>> + gic_handle_fiq_ipi(); >>>>>> +#endif >>>>> >>>>> This hunk is what irritates me. It creates a hard dependency between >>>>> core ARM code and the GIC, and I don't really see how this works with >>>>> multiplatform, where the interrupt controller is not necessarily a GIC. >>>>> In that case, you will die a horrible death. >>>> >>>> I was just about to reassure you that there is no bug here... but then I >>>> read the code. >>>> >>>> gic_handle_fiq_ipi() was *supposed* to do a check to make it safe to >>>> call when there is no gic meaning multi-platform support could be >>>> achieved by calling into multiple handlers. >>>> >>>> It looks like I forgot to write the code that would make this possible. >>>> Maybe I was too disgusted with the approach to implement it correctly. >>>> Looking at this with fresher eyes (I've been having a bit of a break >>>> from FIQ recently) I can see how bad the current approach is. >>>> >>>> >>>>> Why can't we just call handle_arch_irq(), and let the normal handler do >>>>> its thing? You can have a "if (in_nmi())" in there, and call your FIQ >>>>> function. It would at least save us the above problem. >>>> >>>> It should certainly work although it feels odd to reuse the IRQ handler >>>> for FIQ. >>> >>> I can see three options: >>> >>> - (a) Either we have an interrupt controller specific, FIQ only entry >>> point, and we add calls in traps.c: this implies that each driver has >>> to defend itself against spurious calls. >>> >>> - (b) We add a separate handle_arch_fiq() indirection that only deals >>> with FIQ. Much better, but it also means that we have to keep this in >>> sync with arm64, for which the interest is relatively limited (FIQ >>> only works if you have a single security domain like XGene, or for a >>> VM). >>> >>> - (c) We call handle_arch_irq(), and let the interrupt controller code >>> sort the mess. >>> >>> I really hate (a) with a passion, because it litters both the ARM core >>> code with IC specific code *and* introduce some defensive programming >>> in the IC code, which is a waste... >>> >>> Option (b) is nicer, but requires additional work and buy-in from the >>> arm64 maintainers, for a non obvious gain (I quite like the idea of >>> injecting FIQs in a VM though, just for fun...). >>> >>> Option (c) is the simplest, if a little ugly on the side. >>> >>> Thoughts? >> >> For FIQs, do you anticipate handle_arch_irq() having a role like the >> current gic_handle_fiq_ipi(), which is acknowledge an IPI and get out? >> Alternatively it could behave more like its current role for IRQ and >> call into the handlers itself. >> >> The later seems more likely to work out well when I take another look at >> hooking up the perf interrupt. > > Assuming your mention of handle_arch_irq() is actually > handle_arch_fiq(), I'd expect some interesting problems if you try to > handle a Linux interrupt while already handling one, as the core IRQ > code is not designed to be reentrant... Your code works so far because > you have been careful to keep the IRQ code at bay. Putting it back into > the equation is going to be hairy at best. I was actually thinking of option (c) but the question would apply in both cases. To be clear, I agree we cannot call into big piles of irq code from an NMI. We'd have to introduce new NMI-only ways to dispatch FIQs from real hwirqs (SPIs and PPIs). In fact, at present we can't even call into handle_IPI() at the moment (because it will call irq_enter) although we could try to modify things and make that possible. These issues apply whether we have conditional code in handle_arch_irq() or if we introduce handle_arch_fiq(). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/