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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, 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 D1F20ECE58E for ; Mon, 14 Oct 2019 14:52:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B49FB2133F for ; Mon, 14 Oct 2019 14:52:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1733065AbfJNOwI (ORCPT ); Mon, 14 Oct 2019 10:52:08 -0400 Received: from foss.arm.com ([217.140.110.172]:46180 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732457AbfJNOwI (ORCPT ); Mon, 14 Oct 2019 10:52:08 -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 D970E337; Mon, 14 Oct 2019 07:52:07 -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 F16313F68E; Mon, 14 Oct 2019 07:52:06 -0700 (PDT) Date: Mon, 14 Oct 2019 15:52:04 +0100 From: Dave Martin To: Suzuki K Poulose Cc: mark.rutland@arm.com, catalin.marinas@arm.com, will@kernel.org, linux-kernel@vger.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: <20191014145204.GS27757@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <418b0c4b-cbcd-4263-276d-1e9edc5eee0b@arm.com> 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 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. Cheers ---Dave