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=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS 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 505A0C43613 for ; Mon, 24 Jun 2019 14:00:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 30A342145D for ; Mon, 24 Jun 2019 14:00:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729412AbfFXOAE (ORCPT ); Mon, 24 Jun 2019 10:00:04 -0400 Received: from foss.arm.com ([217.140.110.172]:51214 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727170AbfFXOAD (ORCPT ); Mon, 24 Jun 2019 10:00:03 -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 0BD1E344; Mon, 24 Jun 2019 07:00:03 -0700 (PDT) Received: from [10.1.196.72] (e119884-lin.cambridge.arm.com [10.1.196.72]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6A6DC3F71E; Mon, 24 Jun 2019 07:00:00 -0700 (PDT) Subject: Re: [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation To: Will Deacon Cc: linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org, linux-kselftest@vger.kernel.org, Catalin Marinas , Arnd Bergmann , Russell King , Ralf Baechle , Paul Burton , Daniel Lezcano , Thomas Gleixner , Mark Salyzyn , Peter Collingbourne , Shuah Khan , Dmitry Safonov <0x7f454c46@gmail.com>, Rasmus Villemoes , Huw Davies , Shijith Thotton , Andre Przywara References: <20190621095252.32307-1-vincenzo.frascino@arm.com> <20190621095252.32307-5-vincenzo.frascino@arm.com> <20190624133607.GI29497@fuggles.cambridge.arm.com> From: Vincenzo Frascino Message-ID: <13d00991-aaa0-72f9-4556-1c92eceabd07@arm.com> Date: Mon, 24 Jun 2019 14:59:59 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20190624133607.GI29497@fuggles.cambridge.arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kselftest-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org On 24/06/2019 14:36, Will Deacon wrote: > Hi Vincenzo, > > On Fri, Jun 21, 2019 at 10:52:31AM +0100, Vincenzo Frascino wrote: >> To take advantage of the commonly defined vdso interface for >> gettimeofday the architectural code requires an adaptation. >> >> Re-implement the gettimeofday vdso in C in order to use lib/vdso. >> >> With the new implementation arm64 gains support for CLOCK_BOOTTIME >> and CLOCK_TAI. >> >> Cc: Catalin Marinas >> Cc: Will Deacon >> Signed-off-by: Vincenzo Frascino >> Tested-by: Shijith Thotton >> Tested-by: Andre Przywara >> --- >> arch/arm64/Kconfig | 2 + >> arch/arm64/include/asm/vdso/gettimeofday.h | 86 ++++++ >> arch/arm64/include/asm/vdso/vsyscall.h | 53 ++++ >> arch/arm64/include/asm/vdso_datapage.h | 48 --- >> arch/arm64/kernel/asm-offsets.c | 33 +- >> arch/arm64/kernel/vdso.c | 51 +--- >> arch/arm64/kernel/vdso/Makefile | 34 ++- >> arch/arm64/kernel/vdso/gettimeofday.S | 334 --------------------- >> arch/arm64/kernel/vdso/vgettimeofday.c | 28 ++ > > I'm concerned about an apparent semantic change introduced by your patch: > >> +static __always_inline u64 __arch_get_hw_counter(s32 clock_mode) >> +{ >> + u64 res; >> + >> + asm volatile("mrs %0, cntvct_el0" : "=r" (res) :: "memory"); >> + >> + return res; >> +} > > vs: > >> - .macro get_clock_shifted_nsec res, cycle_last, mult >> - /* Read the virtual counter. */ >> - isb >> - mrs x_tmp, cntvct_el0 >> - /* Calculate cycle delta and convert to ns. */ >> - sub \res, x_tmp, \cycle_last >> - /* We can only guarantee 56 bits of precision. */ >> - movn x_tmp, #0xff00, lsl #48 >> - and \res, x_tmp, \res >> - mul \res, \res, \mult >> - /* >> - * Fake address dependency from the value computed from the counter >> - * register to subsequent data page accesses so that the sequence >> - * locking also orders the read of the counter. >> - */ >> - and x_tmp, \res, xzr >> - add vdso_data, vdso_data, x_tmp >> - .endm > > It looks like you're dropping both the preceding ISB (allowing the counter > value to be speculated) and also the subsequent dependency (allowing the > seq lock to be speculated). If I've missed them, apologies, but I couldn't > spot them elsewhere in this patch. > > __arch_get_hw_counter should probably be identical to __arch_counter_get_cntvct > to avoid these problems. I guess we don't need to care about the case where > the counter is unstable, since we'll just disable the vDSO altogether on > such systems? > Oops, I forgot to mirror your patch that introduces this change. I will post a fix in reply to this email. > Will > -- Regards, Vincenzo