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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 A4D90ECE587 for ; Mon, 14 Oct 2019 14:52:17 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 7A76B21835 for ; Mon, 14 Oct 2019 14:52:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="H9e+ArLw" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7A76B21835 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=CaGGzAslIUtr1lPVlMMvuixiFuG90j2z000ZTqOl+9k=; b=H9e+ArLwZBhecM JTIFBjU/qgSWOhk1bMU4bgo7CG7ni8Q4lWB30s7ENefyFVCETbnSpmdjWI+QmTr5HBUnYWXe0QLEJ FfMpVZEZdPUId1I1p/xFHPh9O1K9zUqtPYmLiBaD9CYJZN3bHwsWh5uEcpLph9jbAwX9zJo9tPB4t oVN6Jt3TmUByaX/0cpu7Rx/jt1kJYO3Uvr6D68fAqY1/iMfbohyieHmhP2PTJvAn4eS+AEuDwM9Bl nyqgiohqf2HjRgDI8Az3j871ytA9iXjeZRTXYtjRHQYLyOMsqu89kj4/+1AAjWhCZiuG66rX6J5+M G4U196E1BOFFQO1jGX4A==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1iK1hk-0001um-GN; Mon, 14 Oct 2019 14:52:12 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1iK1hg-0001tw-Ll for linux-arm-kernel@lists.infradead.org; Mon, 14 Oct 2019 14:52:10 +0000 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 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-Disposition: inline In-Reply-To: <418b0c4b-cbcd-4263-276d-1e9edc5eee0b@arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20191014_075208_868844_43843638 X-CRM114-Status: GOOD ( 28.04 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mark.rutland@arm.com, catalin.marinas@arm.com, will@kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel