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 CBD12C53219 for ; Tue, 28 Jul 2026 10:33:26 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.1372341.1619698 (Exim 4.92) (envelope-from ) id 1wof7Q-00050O-W2; Tue, 28 Jul 2026 10:33:04 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 1372341.1619698; Tue, 28 Jul 2026 10:33:04 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1wof7Q-00050G-R1; Tue, 28 Jul 2026 10:33:04 +0000 Received: by outflank-mailman (input) for mailman id 1372341; Tue, 28 Jul 2026 10:33:04 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1wof7Q-00050A-93 for xen-devel@lists.xenproject.org; Tue, 28 Jul 2026 10:33:04 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.96) (envelope-from ) id 1wof7P-00DN1y-0U; Tue, 28 Jul 2026 10:33:02 +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 1wof7O-002qNT-1G; Tue, 28 Jul 2026 10:33:02 +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" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=xenproject.org; s=20200302mail; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date; bh=nB6PbQuKdL6MHngmzjaHTKph6NSfX5lmALrRKO4lcmw=; b=2V4MXmJrzw2FFBQ7aunk3Ro9JU B0D7YtqBjnNTX7j9tfUxHjOPBKDps/eaXCwjGCElL1R/87doqLTFhwLmvpFE28cOrq2uLYlouOoqn ucdWQA7yNUwBCgV4CqrFDlhZI5ka+CKQlk+W9bax6cf6BfCh1scK2H2eCI8PAgqhQfKk=; Date: Tue, 28 Jul 2026 12:32:55 +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 Content-Transfer-Encoding: 8bit In-Reply-To: On Tue, Jul 28, 2026 at 10:24:38AM +0200, Jan Beulich wrote: > On 28.07.2026 10:17, Roger Pau Monné wrote: > > On Tue, Jun 30, 2026 at 04:06:00PM +0200, Jan Beulich wrote: > >> --- 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? > > Aiui it isn't needed here. The compiler can't re-order the two writes, > due to our use of -fno-strict-aliasing. The barrier is there on other > arch-es to avoid re-ordering in hardware. You are the expert in compilers, but it was my understanding that `no-strict-aliasing` only affects the ordering of pointers accesses, but not plain variables. IOW: NOW_good accesses could be reordered because it's not a pointer. Does the compiler consider t can point to &NOW_good and hence it can't be reordered? Thanks, Roger.