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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 8C3C0C10F13 for ; Tue, 16 Apr 2019 14:25:36 +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 1874420693 for ; Tue, 16 Apr 2019 14:25:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1874420693 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 44k72572wSzDqMn for ; Wed, 17 Apr 2019 00:25:33 +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=will.deacon@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 44k70F3BtrzDqJP for ; Wed, 17 Apr 2019 00:23:56 +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 5BB91EBD; Tue, 16 Apr 2019 07:23:54 -0700 (PDT) Received: from fuggles.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id AEE253F59C; Tue, 16 Apr 2019 07:23:51 -0700 (PDT) Date: Tue, 16 Apr 2019 15:23:46 +0100 From: Will Deacon To: Vincenzo Frascino Subject: Re: [PATCH 1/5] arm64: Fix vDSO clock_getres() Message-ID: <20190416142346.GA3803@fuggles.cambridge.arm.com> References: <20190401115152.32751-1-vincenzo.frascino@arm.com> <20190401115152.32751-2-vincenzo.frascino@arm.com> <20190415173521.GA901@arrakis.emea.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.11.1+86 (6f28e57d73f2) () 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 , Catalin Marinas , Heiko Carstens , Paul Mackerras , Martin Schwidefsky , Thomas Gleixner , Vincent Chen , linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org, Greentime Hu Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, Apr 16, 2019 at 01:42:58PM +0100, Vincenzo Frascino wrote: > On 15/04/2019 18:35, Catalin Marinas wrote: > > On Mon, Apr 01, 2019 at 12:51:48PM +0100, Vincenzo Frascino wrote: > >> +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. I think you can ignore the syscall_check, just like we seem to do for CLOCK_REALTIME_COARSE and CLOCK_MONOTONIC_COARSE in clock_gettime(). Will