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 Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 4532EC54F4C for ; Tue, 28 Jul 2026 08:04:00 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.1372204.1619594 (Exim 4.92) (envelope-from ) id 1wocn0-0003vH-LF; Tue, 28 Jul 2026 08:03:50 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 1372204.1619594; Tue, 28 Jul 2026 08:03:50 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1wocn0-0003vA-Ig; Tue, 28 Jul 2026 08:03:50 +0000 Received: by outflank-mailman (input) for mailman id 1372204; Tue, 28 Jul 2026 08:03:49 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1wocmz-0003v2-DE for xen-devel@lists.xenproject.org; Tue, 28 Jul 2026 08:03:49 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.96) (envelope-from ) id 1wocmy-00DJYe-1T; Tue, 28 Jul 2026 08:03:48 +0000 Received: from 224.pool85-54-217.dynamic.orange.es ([85.54.217.224] helo=localhost) by xenbits.xenproject.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.96) (envelope-from ) id 1wocmx-002b0g-2I; Tue, 28 Jul 2026 08:03:48 +0000 X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" Date: Tue, 28 Jul 2026 10:03:40 +0200 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Jan Beulich Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , Julien Grall , Stefano Stabellini , Anthony PERARD , Michal Orzel , Teddy Astie , Bertrand Marquis , Volodymyr Babchuk , Oleksii Kurochko Subject: Re: [PATCH v4 1/3] time: add "NOW() good" indicator Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: On Tue, Jun 30, 2026 at 04:06:00PM +0200, Jan Beulich wrote: > printk_start_of_line() checks for a value of 0 right now. In order to be > able to have NOW() return at least monotonically increasing values, that > needs replacing by an explicit indicator. > > Signed-off-by: Jan Beulich > --- > Arm and RISC-V may want to consider whether their initial get_cycles() > can't be moved yet earlier, such that the indicator also can be set > yet earlier. > --- > v4: Add barriers. > v3: New. > > --- a/xen/arch/arm/time.c > +++ b/xen/arch/arm/time.c > @@ -145,6 +145,8 @@ void __init preinit_xen_time(void) > panic("Timer: Cannot initialize platform timer\n"); > > boot_count = get_cycles(); > + smp_wmb(); > + NOW_good = true; > } > > static void __init init_dt_xen_time(void) > --- a/xen/arch/riscv/time.c > +++ b/xen/arch/riscv/time.c > @@ -87,6 +87,8 @@ void __init preinit_xen_time(void) > panic("%s: ACPI isn't supported\n", __func__); > > boot_clock_cycles = get_cycles(); > + smp_wmb(); > + NOW_good = true; > > /* set_xen_timer must have been set by sbi_init() already */ > ASSERT(set_xen_timer); > --- a/xen/arch/x86/time.c > +++ b/xen/arch/x86/time.c > @@ -2660,6 +2660,7 @@ void __init early_time_init(void) > > set_time_scale(&t->tsc_scale, tmp); > t->stamp.local_tsc = boot_tsc_stamp; > + NOW_good = true; Would you need a barrier here to ensure compiler doesn't re-order the writes? Maybe using ACCESS_ONCE(), or a smp_wmb() like it's used in other arches? Thanks, Roger.