From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C4F7FECE587 for ; Mon, 14 Oct 2019 15:45:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A71DA217D9 for ; Mon, 14 Oct 2019 15:45:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387720AbfJNPpn (ORCPT ); Mon, 14 Oct 2019 11:45:43 -0400 Received: from foss.arm.com ([217.140.110.172]:47346 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729997AbfJNPpn (ORCPT ); Mon, 14 Oct 2019 11:45:43 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 6C8B328; Mon, 14 Oct 2019 08:45:42 -0700 (PDT) Received: from dawn-kernel.cambridge.arm.com (unknown [10.1.197.116]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 9F6F13F718; Mon, 14 Oct 2019 08:45:41 -0700 (PDT) Subject: Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability To: Dave Martin Cc: mark.rutland@arm.com, catalin.marinas@arm.com, will@kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org References: <20191010171517.28782-1-suzuki.poulose@arm.com> <20191010171517.28782-2-suzuki.poulose@arm.com> <20191011113620.GG27757@arm.com> <4ba5c423-4e2a-d810-cd36-32a16ad42c91@arm.com> <20191011142137.GH27757@arm.com> <418b0c4b-cbcd-4263-276d-1e9edc5eee0b@arm.com> <20191014145204.GS27757@arm.com> From: Suzuki K Poulose Message-ID: <12e002e7-42e8-c205-e42c-3348359d2f98@arm.com> Date: Mon, 14 Oct 2019 16:45:40 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0 MIME-Version: 1.0 In-Reply-To: <20191014145204.GS27757@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 14/10/2019 15:52, Dave Martin wrote: > On Fri, Oct 11, 2019 at 06:28:43PM +0100, Suzuki K Poulose wrote: >> >> >> On 11/10/2019 15:21, Dave Martin wrote: >>> On Fri, Oct 11, 2019 at 01:13:18PM +0100, Suzuki K Poulose wrote: > Hi Dave >>>> >>>> On 11/10/2019 12:36, Dave Martin wrote: >>>>> On Thu, Oct 10, 2019 at 06:15:15PM +0100, Suzuki K Poulose wrote: >>>>>> The NO_FPSIMD capability is defined with scope SYSTEM, which implies >>>>>> that the "absence" of FP/SIMD on at least one CPU is detected only >>>>>> after all the SMP CPUs are brought up. However, we use the status >>>>>> of this capability for every context switch. So, let us change >>>>>> the scop to LOCAL_CPU to allow the detection of this capability >>>>>> as and when the first CPU without FP is brought up. >>>>>> >>>>>> Also, the current type allows hotplugged CPU to be brought up without >>>>>> FP/SIMD when all the current CPUs have FP/SIMD and we have the userspace >>>>>> up. Fix both of these issues by changing the capability to >>>>>> BOOT_RESTRICTED_LOCAL_CPU_FEATURE. >>>>>> >>>>>> Fixes: 82e0191a1aa11abf ("arm64: Support systems without FP/ASIMD") >>>>>> Cc: Will Deacon >>>>>> Cc: Mark Rutland >>>>>> Cc: Catalin Marinas >>>>>> Signed-off-by: Suzuki K Poulose >>>>>> --- >>>>>> arch/arm64/kernel/cpufeature.c | 2 +- >>>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c >>>>>> index 9323bcc40a58..0f9eace6c64b 100644 >>>>>> --- a/arch/arm64/kernel/cpufeature.c >>>>>> +++ b/arch/arm64/kernel/cpufeature.c >>>>>> @@ -1361,7 +1361,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = { >>>>>> { >>>>>> /* FP/SIMD is not implemented */ >>>>>> .capability = ARM64_HAS_NO_FPSIMD, >>>>>> - .type = ARM64_CPUCAP_SYSTEM_FEATURE, >>>>>> + .type = ARM64_CPUCAP_BOOT_RESTRICTED_CPU_LOCAL_FEATURE, >>>>> >>>>> ARM64_HAS_NO_FPSIMD is really a disability, not a capability. >>>>> >>>>> Although we have other things that smell like this (CPU errata for >>>>> example), I wonder whether inverting the meaning in the case would >>>>> make the situation easier to understand. >>>> >>>> Yes, it is indeed a disability, more on that below. >>>> >>>>> >>>>> So, we'd have ARM64_HAS_FPSIMD, with a minimum (signed) feature field >>>>> value of 0. Then this just looks like an ARM64_CPUCAP_SYSTEM_FEATURE >>>>> IIUC. We'd just need to invert the sense of the check in >>>>> system_supports_fpsimd(). >>>> >>>> This is particularly something we want to avoid with this patch. We want >>>> to make sure that we have the up-to-date status of the disability right >>>> when it happens. i.e, a CPU without FP/SIMD is brought up. With SYSTEM_FEATURE >>>> you have to wait until we bring all the CPUs up. Also, for HAS_FPSIMD, >>>> you must wait until all the CPUs are up, unlike the negated capability. >>> >>> I don't see why waiting for the random defective early CPU to come up is >>> better than waiting for all the early CPUs to come up and then deciding. >>> >>> Kernel-mode NEON aside, the status of this cap should not matter until >>> we enter userspace for the first time. >>> >>> The only issue is if e.g., crypto drivers that can use kernel-mode NEON >>> probe for it before all early CPUs are up, and so cache the wrong >>> decision. The current approach doesn't cope with that anyway AFAICT. >> >> This approach does in fact. With LOCAL_CPU scope, the moment a defective >> CPU turns up, we mark the "capability" and thus the kernel cannot use >> the neon then onwards, unlike the existing case where we have time till >> we boot all the CPUs (even when the boot CPU may be defective). > > I guess that makes sense. > > I'm now wondering what happens if anything tries to use kernel-mode NEON > before SVE is initialised -- which doesn't happen until cpufeatures > configures the system features. > > I don't think your proposed change makes anything worse here, but it may > need looking into. We could throw in a WARN_ON() in kernel_neon() to make sure that the SVE is initialised ? Suzuki