From nobody Tue Feb 10 00:57:44 2026 Delivered-To: importer@patchew.org Received-SPF: pass (zohomail.com: domain of lists.xenproject.org designates 192.237.175.120 as permitted sender) client-ip=192.237.175.120; envelope-from=xen-devel-bounces@lists.xenproject.org; helo=lists.xenproject.org; Authentication-Results: mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of lists.xenproject.org designates 192.237.175.120 as permitted sender) smtp.mailfrom=xen-devel-bounces@lists.xenproject.org; dmarc=pass(p=quarantine dis=none) header.from=suse.com ARC-Seal: i=1; a=rsa-sha256; t=1617270928; cv=none; d=zohomail.com; s=zohoarc; b=ilYz+6ucFxC5jB1ph/bcklx0Gv8N5ob06vzbVfziS4huhNFxtfeHyFOB8CC1dn6zo7lTkP+PGMB+MhYwaVv+cNUOReqq6+IdeWiAP7i1swGcg2miHOHreBHXetMIBNIx44YupkQfFMXJru9KqTA7wlERBNvriVElv/UY19KOyNI= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=zohomail.com; s=zohoarc; t=1617270928; h=Content-Type:Content-Transfer-Encoding:Cc:Date:From:In-Reply-To:List-Subscribe:List-Post:List-Id:List-Help:List-Unsubscribe:MIME-Version:Message-ID:References:Sender:Subject:To; bh=/4bBVDd6Hj6a8b8XlnvfpgQxmTCGoW9DQonxX1lum1U=; b=b8qlcpHvDj/r4YZSdPRN6UbKJSh1mOCkcGSrc33486GCmqoPaB9rsthRDU5U66ipD/QcMErBMcmsSU/IxItL3Aw26xQ3v9YdqFpHUs2wJwaXp6NwCJl0N1LUFrHsm/Xwtnnjq22wnMqk3vezi72Ke1Xu+5uPeh6RtTe65pAlArk= ARC-Authentication-Results: i=1; mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of lists.xenproject.org designates 192.237.175.120 as permitted sender) smtp.mailfrom=xen-devel-bounces@lists.xenproject.org; dmarc=pass header.from= (p=quarantine dis=none) header.from= Return-Path: Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) by mx.zohomail.com with SMTPS id 1617270928367506.46814388792393; Thu, 1 Apr 2021 02:55:28 -0700 (PDT) Received: from list by lists.xenproject.org with outflank-mailman.104267.199229 (Exim 4.92) (envelope-from ) id 1lRu2k-0006df-Cj; Thu, 01 Apr 2021 09:55:14 +0000 Received: by outflank-mailman (output) from mailman id 104267.199229; Thu, 01 Apr 2021 09:55:14 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lRu2k-0006dY-9h; Thu, 01 Apr 2021 09:55:14 +0000 Received: by outflank-mailman (input) for mailman id 104267; Thu, 01 Apr 2021 09:55:12 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lRu2i-0006dG-Gy for xen-devel@lists.xenproject.org; Thu, 01 Apr 2021 09:55:12 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 4a3658ca-cfbc-451f-9886-16fc96a54119; Thu, 01 Apr 2021 09:55:11 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 04D25B032; Thu, 1 Apr 2021 09:55:11 +0000 (UTC) X-Outflank-Mailman: Message body and most headers restored to incoming version 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" X-Inumbo-ID: 4a3658ca-cfbc-451f-9886-16fc96a54119 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1617270911; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=/4bBVDd6Hj6a8b8XlnvfpgQxmTCGoW9DQonxX1lum1U=; b=tMyzVyeb/17TZQ3RGutErR3hQvhLIo/di2rDhns5u9ADhQOQwn82UKhNTJ7TvTs6QzDZs0 HkEPXkeflxHNeOkL6+gRVOGiOLffJAxyv6z119bgPAg8CZNZTSZi/A2Fv1UJBDTFkKdWXP 73AhBd+cg2PvrtSrjiwQC+Qb0u/wEwM= Subject: [PATCH v4 3/3] x86/time: avoid reading the platform timer in rendezvous functions From: Jan Beulich To: "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= References: Message-ID: Date: Thu, 1 Apr 2021 11:55:10 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.9.0 MIME-Version: 1.0 In-Reply-To: Content-Language: en-US Content-Transfer-Encoding: quoted-printable X-ZohoMail-DKIM: pass (identity @suse.com) Content-Type: text/plain; charset="utf-8" Reading the platform timer isn't cheap, so we'd better avoid it when the resulting value is of no interest to anyone. The consumer of master_stime, obtained by time_calibration_{std,tsc}_rendezvous() and propagated through this_cpu(cpu_calibration), is local_time_calibration(). With CONSTANT_TSC the latter function uses an early exit path, which doesn't explicitly use the field. While this_cpu(cpu_calibration) (including the master_stime field) gets propagated to this_cpu(cpu_time).stamp on that path, both structures' fields get consumed only by the !CONSTANT_TSC logic of the function. Signed-off-by: Jan Beulich Acked-by: Roger Pau Monn=C3=A9 --- v4: New. --- I realize there's some risk associated with potential new uses of the field down the road. What would people think about compiling time.c a 2nd time into a dummy object file, with a conditional enabled to force assuming CONSTANT_TSC, and with that conditional used to suppress presence of the field as well as all audited used of it (i.e. in particular that large part of local_time_calibration())? Unexpected new users of the field would then cause build time errors. --- a/xen/arch/x86/time.c +++ b/xen/arch/x86/time.c @@ -52,6 +52,7 @@ unsigned long pit0_ticks; struct cpu_time_stamp { u64 local_tsc; s_time_t local_stime; + /* Next field unconditionally valid only when !CONSTANT_TSC. */ s_time_t master_stime; }; =20 @@ -1702,7 +1703,7 @@ static void time_calibration_tsc_rendezv * iteration. */ r->master_tsc_stamp =3D r->max_tsc_stamp; - else if ( i =3D=3D 0 ) + else if ( !boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && i =3D=3D = 0 ) r->master_stime =3D read_platform_stime(NULL); =20 atomic_inc(&r->semaphore); @@ -1776,8 +1777,11 @@ static void time_calibration_std_rendezv { while ( atomic_read(&r->semaphore) !=3D (total_cpus - 1) ) cpu_relax(); - r->master_stime =3D read_platform_stime(NULL); - smp_wmb(); /* write r->master_stime /then/ signal */ + if ( !boot_cpu_has(X86_FEATURE_CONSTANT_TSC) ) + { + r->master_stime =3D read_platform_stime(NULL); + smp_wmb(); /* write r->master_stime /then/ signal */ + } atomic_inc(&r->semaphore); } else