From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932195AbdC1Os7 (ORCPT ); Tue, 28 Mar 2017 10:48:59 -0400 Received: from foss.arm.com ([217.140.101.70]:50260 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752724AbdC1Os4 (ORCPT ); Tue, 28 Mar 2017 10:48:56 -0400 Subject: Re: [PATCH v2 06/18] arm64: arch_timer: Add infrastructure for multiple erratum detection methods To: Daniel Lezcano References: <20170320174829.28182-1-marc.zyngier@arm.com> <20170320174829.28182-7-marc.zyngier@arm.com> <20170322154114.GE30499@mai> <20170323173030.GA24630@mai> <5e8faa45-bb1b-fd9b-f34f-93ce5babb0a3@arm.com> <20170327075628.GE24630@mai> <20170328133438.GB2123@mai> <20170328143633.GC2123@mai> Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Mark Rutland , Catalin Marinas , Will Deacon , Scott Wood , Hanjun Guo , Ding Tianhong , dann frazier From: Marc Zyngier Organization: ARM Ltd Message-ID: Date: Tue, 28 Mar 2017 15:48:23 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Icedove/45.6.0 MIME-Version: 1.0 In-Reply-To: <20170328143633.GC2123@mai> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 28/03/17 15:36, Daniel Lezcano wrote: > On Tue, Mar 28, 2017 at 03:07:52PM +0100, Marc Zyngier wrote: > > [ ... ] > >>>>> -bool arch_timer_check_global_cap_erratum(const struct arch_timer_erratum_workaround *wa, >>>>> - const void *arg) >>>>> +bool arch_timer_check_cap_erratum(const struct arch_timer_erratum_workaround *wa, >>>>> + const void *arg) >>>>> { >>>>> - return cpus_have_cap((uintptr_t)wa->id); >>>>> + return cpus_have_cap((uintptr_t)wa->id) | this_cpu_has_cap((uintptr_t)wa->id); >>>> >>>> Not quite. Here, you're making all capability-based errata to be be >>>> global (if a single CPU in the system has a capability, then by >>>> transitivity cpus_have_cap returns true). If that's a big-little system, >>>> you end-up applying the workaround to all CPUs, including those unaffected. >>>> >>>> I'd rather drop cpus_have_cap altogether and rely on individual CPU >>>> matching (since we don't have a need for a global capability erratum >>>> handling yet). >>> >>> Ok, thanks. >> >> Quick update. I've just implemented this, and found out that getting rid >> of local/global has an unfortunate effect: >> >> Since we only probe the global errata (using ACPI for example) on the >> boot CPU path, we lose propagation of the erratum across the secondary >> CPUs. One way of solving this is to convert the secondary boot path to >> be aware of DT vs ACPI vs detection method of the month. Which isn't >> easy, since by the time we boot secondary CPUs, we don't have the >> pointers to the various ACPI tables anymore. Also, assuming we were >> careful and saved the pointers, the tables may have been unmapped. Fun. > > My proposal was supposed to prevent that. The detecion is done in the > subsystems, ACPI detects ACPI errata, DT detects DT errata and CPU detects CPU > errata. The drivers get the errata and enable the workaround. The id > association <-> errata self contains errata types (void *, char *, int). So > everything can be done in a CPU basis without local / global dance. I'm sorry, but it feels like a Jumbo-Jet sized hammer to try and squash a fly (I'm staying away from the frozen shark metaphor here). You're willing to add a whole list of things with private ids that need matching to kill a flag? I don't think this buys us anything but extra complexity and another maintenance headache. Thanks, M. -- Jazz is not dead. It just smells funny... From mboxrd@z Thu Jan 1 00:00:00 1970 From: marc.zyngier@arm.com (Marc Zyngier) Date: Tue, 28 Mar 2017 15:48:23 +0100 Subject: [PATCH v2 06/18] arm64: arch_timer: Add infrastructure for multiple erratum detection methods In-Reply-To: <20170328143633.GC2123@mai> References: <20170320174829.28182-1-marc.zyngier@arm.com> <20170320174829.28182-7-marc.zyngier@arm.com> <20170322154114.GE30499@mai> <20170323173030.GA24630@mai> <5e8faa45-bb1b-fd9b-f34f-93ce5babb0a3@arm.com> <20170327075628.GE24630@mai> <20170328133438.GB2123@mai> <20170328143633.GC2123@mai> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 28/03/17 15:36, Daniel Lezcano wrote: > On Tue, Mar 28, 2017 at 03:07:52PM +0100, Marc Zyngier wrote: > > [ ... ] > >>>>> -bool arch_timer_check_global_cap_erratum(const struct arch_timer_erratum_workaround *wa, >>>>> - const void *arg) >>>>> +bool arch_timer_check_cap_erratum(const struct arch_timer_erratum_workaround *wa, >>>>> + const void *arg) >>>>> { >>>>> - return cpus_have_cap((uintptr_t)wa->id); >>>>> + return cpus_have_cap((uintptr_t)wa->id) | this_cpu_has_cap((uintptr_t)wa->id); >>>> >>>> Not quite. Here, you're making all capability-based errata to be be >>>> global (if a single CPU in the system has a capability, then by >>>> transitivity cpus_have_cap returns true). If that's a big-little system, >>>> you end-up applying the workaround to all CPUs, including those unaffected. >>>> >>>> I'd rather drop cpus_have_cap altogether and rely on individual CPU >>>> matching (since we don't have a need for a global capability erratum >>>> handling yet). >>> >>> Ok, thanks. >> >> Quick update. I've just implemented this, and found out that getting rid >> of local/global has an unfortunate effect: >> >> Since we only probe the global errata (using ACPI for example) on the >> boot CPU path, we lose propagation of the erratum across the secondary >> CPUs. One way of solving this is to convert the secondary boot path to >> be aware of DT vs ACPI vs detection method of the month. Which isn't >> easy, since by the time we boot secondary CPUs, we don't have the >> pointers to the various ACPI tables anymore. Also, assuming we were >> careful and saved the pointers, the tables may have been unmapped. Fun. > > My proposal was supposed to prevent that. The detecion is done in the > subsystems, ACPI detects ACPI errata, DT detects DT errata and CPU detects CPU > errata. The drivers get the errata and enable the workaround. The id > association <-> errata self contains errata types (void *, char *, int). So > everything can be done in a CPU basis without local / global dance. I'm sorry, but it feels like a Jumbo-Jet sized hammer to try and squash a fly (I'm staying away from the frozen shark metaphor here). You're willing to add a whole list of things with private ids that need matching to kill a flag? I don't think this buys us anything but extra complexity and another maintenance headache. Thanks, M. -- Jazz is not dead. It just smells funny...