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=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED 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 CFF1EC10F13 for ; Tue, 16 Apr 2019 12:45:05 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (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 90288205ED for ; Tue, 16 Apr 2019 12:45:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 90288205ED Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from lists.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 44k4p56SLszDqLM for ; Tue, 16 Apr 2019 22:45:01 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=arm.com (client-ip=217.140.101.70; helo=foss.arm.com; envelope-from=vincenzo.frascino@arm.com; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=arm.com Received: from foss.arm.com (foss.arm.com [217.140.101.70]) by lists.ozlabs.org (Postfix) with ESMTP id 44k4lt1Dr6zDqCp for ; Tue, 16 Apr 2019 22:43:05 +1000 (AEST) 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 77EEAEBD; Tue, 16 Apr 2019 05:43:02 -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 ECF423F706; Tue, 16 Apr 2019 05:42:59 -0700 (PDT) Subject: Re: [PATCH 1/5] arm64: Fix vDSO clock_getres() To: Catalin Marinas References: <20190401115152.32751-1-vincenzo.frascino@arm.com> <20190401115152.32751-2-vincenzo.frascino@arm.com> <20190415173521.GA901@arrakis.emea.arm.com> From: Vincenzo Frascino Message-ID: Date: Tue, 16 Apr 2019 13:42:58 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190415173521.GA901@arrakis.emea.arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, Shuah Khan , Arnd Bergmann , Heiko Carstens , Will Deacon , Paul Mackerras , Greentime Hu , Martin Schwidefsky , Thomas Gleixner , Vincent Chen , linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Hi Catalin, On 15/04/2019 18:35, Catalin Marinas wrote: > On Mon, Apr 01, 2019 at 12:51:48PM +0100, Vincenzo Frascino wrote: >> diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c >> index 2d419006ad43..47ba72345739 100644 >> --- a/arch/arm64/kernel/vdso.c >> +++ b/arch/arm64/kernel/vdso.c >> @@ -245,6 +245,8 @@ void update_vsyscall(struct timekeeper *tk) >> vdso_data->cs_shift = tk->tkr_mono.shift; >> } >> >> + vdso_data->hrtimer_res = hrtimer_resolution; >> + >> smp_wmb(); >> ++vdso_data->tb_seq_count; >> } >> diff --git a/arch/arm64/kernel/vdso/gettimeofday.S b/arch/arm64/kernel/vdso/gettimeofday.S >> index c39872a7b03c..7a2cd2f8e13a 100644 >> --- a/arch/arm64/kernel/vdso/gettimeofday.S >> +++ b/arch/arm64/kernel/vdso/gettimeofday.S >> @@ -296,32 +296,35 @@ ENDPROC(__kernel_clock_gettime) >> /* int __kernel_clock_getres(clockid_t clock_id, struct timespec *res); */ >> ENTRY(__kernel_clock_getres) >> .cfi_startproc >> + adr vdso_data, _vdso_data >> cmp w0, #CLOCK_REALTIME >> ccmp w0, #CLOCK_MONOTONIC, #0x4, ne >> ccmp w0, #CLOCK_MONOTONIC_RAW, #0x4, ne >> - b.ne 1f >> + b.ne 2f >> >> - ldr x2, 5f >> - b 2f >> -1: >> +1: /* Get hrtimer_res */ >> + seqcnt_acquire >> + syscall_check fail=5f >> + ldr x2, [vdso_data, #CLOCK_REALTIME_RES] >> + seqcnt_check fail=1b >> + b 3f >> +2: > > We talked briefly but I'm still confused why we need the fallback to the > syscall here if archdata.vdso_direct is false. Is it because if the > timer driver code sets vdso_direct to false, we don't don't support > highres timers? If my understanding is correct, you may want to move the > hrtimer_res setting in update_vsyscall() to the !use_syscall block. > Ok, so let me try to provide more details on what I mentioned yesterday: - clock_getres syscall follows the rules of what defined in posix-timers.c - based on the clock_id that, for this purpose, can be separated in coarse and non-coarse calls either posix_get_coarse_res() or posix_get_hrtimer_res(). - if clock id is set to a coarse clock and posix_get_coarse_res() is invoked, happens what follows: static int posix_get_coarse_res(const clockid_t which_clock, struct timespec64 *tp) { *tp = ktime_to_timespec64(KTIME_LOW_RES); return 0; } Note that since CONFIG_1HZ seems not supported (jiffies.h) by the kernel in this case we do not need rounding in our vDSO implementation. - if clock id is set to non-coarse and posix_get_hrtimer_res() is invoked, happens the following: static int posix_get_hrtimer_res(clockid_t which_clock, struct timespec64 *tp) { tp->tv_sec = 0; tp->tv_nsec = hrtimer_resolution; return 0; } hrtimer_resolution can be high res or low res depending on the call of hrtimer_switch_to_hres(). For us the only way to preserve the correct value is to keep it in the vdso data page. - The assembly code mimics exactly the same behaviour detailed above, with one difference: the one related to the use_syscall parameter which is specific to arm64. The use_syscall parameter is set by arm_arch_timer and consumed by update_vsyscall(). To mirror what update_vsyscall does in update_vsyscall() I check "syscall_check fail=5f" in clock_getres vdso function. Said that, even if functionally it is the same thing, I think it is logically more correct to have hrtimer_res setting inside the !use_syscall block, hence I am going to change it in the next iteration. Please let me know your thoughts. -- Regards, Vincenzo