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 D7D9AC10F14 for ; Tue, 15 Oct 2019 10:25:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ABB3E20854 for ; Tue, 15 Oct 2019 10:25:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730455AbfJOKZD (ORCPT ); Tue, 15 Oct 2019 06:25:03 -0400 Received: from foss.arm.com ([217.140.110.172]:34562 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728185AbfJOKZD (ORCPT ); Tue, 15 Oct 2019 06:25:03 -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 630E428; Tue, 15 Oct 2019 03:25:02 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5F1AA3F68E; Tue, 15 Oct 2019 03:25:01 -0700 (PDT) Date: Tue, 15 Oct 2019 11:24:59 +0100 From: Dave Martin To: Ard Biesheuvel Cc: Mark Rutland , Suzuki Poulose , Catalin Marinas , "linux-kernel@vger.kernel.org" , "will@kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability Message-ID: <20191015102459.GV27757@arm.com> 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> <12e002e7-42e8-c205-e42c-3348359d2f98@arm.com> <20191014155009.GM24047@e103592.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 14, 2019 at 06:57:30PM +0200, Ard Biesheuvel wrote: > On Mon, 14 Oct 2019 at 17:50, Dave P Martin wrote: > > > > On Mon, Oct 14, 2019 at 04:45:40PM +0100, Suzuki K Poulose wrote: > > > > > > > > > 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 ? > > > > Could do, at least as an experiment. > > > > Ard, do you have any thoughts on this? > > > > All in-kernel NEON code checks whether the NEON is usable, so I'd > expect that check to return 'false' if it is too early in the boot for > the NEON to be used at all. My concern is that the check may be done once, at probe time, for crypto drivers. If probing happens before system_supports_fpsimd() has stabilised, we may be stuck with the wrong probe decision. So: are crypto drivers and kernel_mode_neon() users definitely only probed _after_ all early CPUs are up? Cheers ---Dave