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=-5.5 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_PASS,USER_AGENT_MUTT 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 8A0A8C282C2 for ; Wed, 13 Feb 2019 17:04:30 +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 4EA00222CE for ; Wed, 13 Feb 2019 17:04:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="q20OLz52" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4EA00222CE 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=ii7Jrl66QvsGE0xBGXj65m8VOI+6EB3XxFheRdieDbM=; b=q20OLz52s0a53L TnQ1gLZljc4er/nltleKfU7Z8oK3k/AqV6oPPbqsPjbIrxuUdM9+Kanqk1jBGtiL/Vb/THF26tDr2 21zUEul3vbvrtFthA0UeUwc0aVQ3FqYnKu42fI+DGoXtVJT8XW5pfBuq476N0jc4BlX54LxrIIDuo htd8quwvcZD4j3+PSx96sGSa6r/YW4lkjePmjs+GVbvSLQ8023C4cDPPIzAPRC6RyDdRLflU68BAa bfg/VembXvdSpXKEHyxYKcqG9YXjNl03jF9iten3D1xpQ8Q08dUmP3bxN2lmjb+ADE/We+meHKe7f sYTS2kVtZWNakBqV3M4w==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1gtxxU-0005gM-0E; Wed, 13 Feb 2019 17:04:28 +0000 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70] helo=foss.arm.com) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1gtxxQ-0005f8-94 for linux-arm-kernel@lists.infradead.org; Wed, 13 Feb 2019 17:04:26 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 1A2E6A78; Wed, 13 Feb 2019 09:04:22 -0800 (PST) Received: from brain-police (usa-sjc-mx-foss1.foss.arm.com [217.140.101.70]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id ED1793F675; Wed, 13 Feb 2019 09:04:18 -0800 (PST) Date: Wed, 13 Feb 2019 17:04:14 +0000 From: Will Deacon To: Thomas Gleixner Subject: Re: [PATCH v2 06/28] kernel: Define gettimeofday vdso common code Message-ID: <20190213170413.GE6346@brain-police> References: <20181129170530.37789-1-vincenzo.frascino@arm.com> <20181129170530.37789-7-vincenzo.frascino@arm.com> <20181207175321.GA11430@edgewater-inn.cambridge.arm.com> <20190208173539.GD24375@fuggles.cambridge.arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190213_090424_328380_0ABFE470 X-CRM114-Status: GOOD ( 18.25 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, Arnd Bergmann , marc.zyngier@arm.com, Catalin Marinas , Daniel Lezcano , Russell King , Ralf Baechle , Mark Salyzyn , Paul Burton , Vincenzo Frascino , Peter Collingbourne , 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 [+Marc for the arch timer] On Fri, Feb 08, 2019 at 08:30:25PM +0100, Thomas Gleixner wrote: > On Fri, 8 Feb 2019, Thomas Gleixner wrote: > > On Fri, 8 Feb 2019, Will Deacon wrote: > > > On Fri, Dec 07, 2018 at 05:53:21PM +0000, Will Deacon wrote: > > > > Anyway, moving the counter read into the protected region is a little fiddly > > > > because the memory barriers we have in there won't give us the ordering we > > > > need. We'll instead need to do something nasty, like create a dependency > > > > from the counter read to the read of the seqlock: > > > > > > > > Maybe the untested crufty hack below, although this will be a nightmare to > > > > implement in C. > > How is the in kernel ktime_get() correctness guaranteed then? Luck. I think we'll have to introduce a dummy dependent stack read into our counter accessor so that it's ordered by the smp_rmb(). Example diff below, which I'll roll into a proper patch series later on. Will --->8 diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h index f2a234d6516c..bd55b4373700 100644 --- a/arch/arm64/include/asm/arch_timer.h +++ b/arch/arm64/include/asm/arch_timer.h @@ -150,8 +150,24 @@ static inline void arch_timer_set_cntkctl(u32 cntkctl) static inline u64 arch_counter_get_cntpct(void) { + u64 cnt, tmp; + isb(); - return arch_timer_reg_read_stable(cntpct_el0); + cnt = arch_timer_reg_read_stable(cntpct_el0); + + /* + * This insanity brought to you by speculative, out-of-order system + * register reads, sequence locks and Thomas Gleixner. + * + * http://lists.infradead.org/pipermail/linux-arm-kernel/2019-February/631195.html + */ + asm volatile("eor %0, %1, %1\n" + "add %0, sp, %0\n" + "ldr xzr, [%0]" + : "=r" (tmp) + : "r" (cnt) + : "memory"); + return cnt; } static inline u64 arch_counter_get_cntvct(void) _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel