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 9F0B8C636D7 for ; Tue, 21 Feb 2023 05:52:11 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.498520.769391 (Exim 4.92) (envelope-from ) id 1pULYn-0000mD-8c; Tue, 21 Feb 2023 05:51:29 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 498520.769391; Tue, 21 Feb 2023 05:51:29 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1pULYn-0000m6-64; Tue, 21 Feb 2023 05:51:29 +0000 Received: by outflank-mailman (input) for mailman id 498520; Tue, 21 Feb 2023 05:51:27 +0000 Received: from se1-gles-flk1-in.inumbo.com ([94.247.172.50] helo=se1-gles-flk1.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1pULYl-0000lx-AE for xen-devel@lists.xenproject.org; Tue, 21 Feb 2023 05:51:27 +0000 Received: from bumble.birch.relay.mailchannels.net (bumble.birch.relay.mailchannels.net [23.83.209.25]) by se1-gles-flk1.inumbo.com (Halon) with ESMTPS id c4557ade-b1ab-11ed-93b5-47a8fe42b414; Tue, 21 Feb 2023 06:51:23 +0100 (CET) Received: from relay.mailchannels.net (localhost [127.0.0.1]) by relay.mailchannels.net (Postfix) with ESMTP id 7126E6C14D2 for ; Tue, 21 Feb 2023 05:51:20 +0000 (UTC) Received: from pdx1-sub0-mail-a304.dreamhost.com (unknown [127.0.0.6]) (Authenticated sender: dreamhost) by relay.mailchannels.net (Postfix) with ESMTPA id EEA9B6C13D7 for ; Tue, 21 Feb 2023 05:51:19 +0000 (UTC) Received: from pdx1-sub0-mail-a304.dreamhost.com (pop.dreamhost.com [64.90.62.162]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384) by 100.107.134.126 (trex/6.7.1); Tue, 21 Feb 2023 05:51:20 +0000 Received: from kmjvbox (c-76-102-200-71.hsd1.ca.comcast.net [76.102.200.71]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: kjlx@templeofstupid.com) by pdx1-sub0-mail-a304.dreamhost.com (Postfix) with ESMTPSA id 4PLT1M3nCPz4f for ; Mon, 20 Feb 2023 21:51:19 -0800 (PST) Received: from johansen (uid 1000) (envelope-from kjlx@templeofstupid.com) id e00f5 by kmjvbox (DragonFly Mail Agent v0.12); Mon, 20 Feb 2023 21:51:17 -0800 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: c4557ade-b1ab-11ed-93b5-47a8fe42b414 X-Sender-Id: dreamhost|x-authsender|kjlx@templeofstupid.com ARC-Seal: i=1; s=arc-2022; d=mailchannels.net; t=1676958680; a=rsa-sha256; cv=none; b=P/LyeC5VJy2k15smy91wrTaCZPsx3iu4tVq3lIjboluvHkv0xcq39GNu0kfDijFfsHLf0p ULIXZxwyQYkAClvopN00WuZhlRclnlvezipw0h3qSBxH4CugmjEmg3nlgi/5huq/vZeqh7 SeBDZDW1b6UcbdYxooVA4nW1yaj6/1C0/10shj4EHPgzNzKAb43ZBPpPoZt/7PPlmWs8gd kc+BH/vg9IERTZJw9pcdkU7jCyxp7yLrcXSMvDCZdRnqWEIWbqH7QhET9xajN/r8gekLmY lzCGnaL7vEYExdnOSsF3nea5sN3A4YjB9o1Bh8VYS8oQRs6YedqRO1UsafUStQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=mailchannels.net; s=arc-2022; t=1676958680; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references:dkim-signature; bh=4Ryms4gHoy/MGnNprWGKzCdsTkn+W4ai7vFOzO+3AGw=; b=3P6hxWtKbaqG76Fh6IYONPJSCJYuOVJv2GC1nxO/in/eyXs9LZCqPLKd3d6SSK6cp99bUQ UFLoukSPM3nx4mDYL+Z3Usehjh1VBPC0v8ZD2Ob2QZZsjAnmtF/ra3hWek2fW55Wf0w3y4 6fyd/PPoOGnWipC18REV7qgKVdMnNtDgYKklX30BC615KXDv0HN5L5XsxFu4I4o9YkF6Uz Szn5NnDbHQDbhKYyQj5uT3dvqfMlp0xzB+GBfg8jS4nxF/Sa/gxomFi03+SNeJEk+aHFCq qRkRP/eYRlzV8M+eQoCo4IWqx2ZLpFK8BOFI5QNL3vdTAp1ztpaYlsW9UgI7Sw== ARC-Authentication-Results: i=1; rspamd-5db48964c-9hc9p; auth=pass smtp.auth=dreamhost smtp.mailfrom=kjlx@templeofstupid.com X-Sender-Id: dreamhost|x-authsender|kjlx@templeofstupid.com X-MC-Relay: Neutral X-MailChannels-SenderId: dreamhost|x-authsender|kjlx@templeofstupid.com X-MailChannels-Auth-Id: dreamhost X-Zesty-Invention: 543ef75e49736803_1676958680241_3740235661 X-MC-Loop-Signature: 1676958680241:2041868574 X-MC-Ingress-Time: 1676958680241 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=templeofstupid.com; s=dreamhost; t=1676958679; bh=4Ryms4gHoy/MGnNprWGKzCdsTkn+W4ai7vFOzO+3AGw=; h=Date:From:To:Cc:Subject:Content-Type; b=ODo7CVzs5sFOwiyqDH58lvqtbRhN/pp+GDD4QD1mQcWjhYGj7KIwGzNCXNv05epkH AHrXXdv5ri7R5VdJHLYFtSuAw+zt7Zc5ZYr9It1N2KHKsUuicchc0+223SRciI5eHi bcysQHndCS0eGw34Z4+B0DZFd4J+TGPvdwlX+YZ4= Date: Mon, 20 Feb 2023 21:51:17 -0800 From: Krister Johansen To: Thomas Gleixner Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, Juergen Gross , Jan Beulich , Boris Ostrovsky , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Marcelo Tosatti , Anthony Liguori , David Reaver , Brendan Gregg Subject: Re: [PATCH linux-next 2/2] x86/xen/time: cleanup xen_tsc_safe_clocksource Message-ID: <20230221055117.GA1934@templeofstupid.com> References: <87h6vgov2p.ffs@tglx> <20230221041440.GA1934@templeofstupid.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230221041440.GA1934@templeofstupid.com> On Mon, Feb 20, 2023 at 08:14:40PM -0800, Krister Johansen wrote: > On Mon, Feb 20, 2023 at 11:01:18PM +0100, Thomas Gleixner wrote: > > On Mon, Feb 20 2023 at 09:17, Krister Johansen wrote: > > > @@ -495,8 +496,7 @@ static int __init xen_tsc_safe_clocksource(void) > > > /* Leaf 4, sub-leaf 0 (0x40000x03) */ > > > cpuid_count(xen_cpuid_base() + 3, 0, &eax, &ebx, &ecx, &edx); > > > > > > - /* tsc_mode = no_emulate (2) */ > > > - if (ebx != 2) > > > + if (ebx != XEN_CPUID_TSC_MODE_NEVER_EMULATE) > > > return 0; > > > > > > return 1; > > > > What about removing more stupidity from that function? > > > > static bool __init xen_tsc_safe_clocksource(void) > > { > > u32 eax, ebx. ecx, edx; > > > > /* Leaf 4, sub-leaf 0 (0x40000x03) */ > > cpuid_count(xen_cpuid_base() + 3, 0, &eax, &ebx, &ecx, &edx); > > > > return ebx == XEN_CPUID_TSC_MODE_NEVER_EMULATE; > > } > > I'm all for simplifying. I'm happy to clean up that return to be more > idiomatic. I was under the impression, perhaps mistaken, though, that > the X86_FEATURE_CONSTANT_TSC, X86_FEATURE_NONSTOP_TSC, and > check_tsc_unstable() checks were actually serving a purpose: to ensure > that we don't rely on the tsc in environments where it's being emulated > and the OS would be better served by using a PV clock. Specifically, > kvmclock_init() makes a very similar set of checks that I also thought > were load-bearing. Bah, what I meant to say was emulated, unstable, or otherwise unsuitable for use as a clocksource. IOW, even if TSC_MODE_NEVER_EMULATE is set, it's possible that a user is attempting a migration from a cpu that's not invariant, and we'd still want to check for that case and fall back to a PV clocksource, correct? -K