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=1.7 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, FSL_HELO_FAKE,MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=no 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 0BF66C43441 for ; Mon, 19 Nov 2018 08:39:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C177820871 for ; Mon, 19 Nov 2018 08:39:02 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="NkfRWYuP" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C177820871 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727198AbeKSTB5 (ORCPT ); Mon, 19 Nov 2018 14:01:57 -0500 Received: from mail-wm1-f46.google.com ([209.85.128.46]:51266 "EHLO mail-wm1-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726311AbeKSTB5 (ORCPT ); Mon, 19 Nov 2018 14:01:57 -0500 Received: by mail-wm1-f46.google.com with SMTP id w7-v6so4364338wmc.1; Mon, 19 Nov 2018 00:38:59 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=zyUNGkUCzTLUbl0hFBf1C0LnwA7UI9OL1j1upJ5ly1M=; b=NkfRWYuPlQ66FVOj/WdXZGRwCsVKq4KUzlqji2JSKJIJGo7tkJDSrgbFL+3oCfOkET OQMz8W63XJXRLkExNmrWP5tij1O8oflP+XlvTq64k4iRr2DbhZOlinLIAHZewv28tECC EYyH96BMOxu/osqVf38fydNwBpZtGdNCEcQOj9nkHWtujoRmixEx49zxCNnqzb5QMBPE XYJ0OwDTuXsWxuYQdDOFC032lOKeKVkZ0O73UhGJwC3w+IRh29R+McbBcm5q3gCrmDkO 7mi47fhhIvw1OBFtfnbPVL7E3JIZrab24WmGhceU2BkwZTcc9RE86j/7Zvs4SLtdBT1K QTuQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:from:to:cc:subject:message-id :references:mime-version:content-disposition:in-reply-to:user-agent; bh=zyUNGkUCzTLUbl0hFBf1C0LnwA7UI9OL1j1upJ5ly1M=; b=P+bmb/hLFNI6f3AKGBv5776r5pvS9CkSnWFPyO+46iWindDrvXLevkT+/ggyQ79VcD VFoyxehMXuNpiuVwhpNeegFWTo3femk9ffciJWtDcEI321S7z46DArPpRYM2S+NSoHJd +Y9FL23FpiwZt8/Jv1F6fSW1hqUo2MqjEOgM9MYgnWiB/LycFyPkDn07CDe6UbQKnUsH L4/rcMfq2ObRVulpazFQqJI9aGMw31Z5CUECvGlkWsAmiVGH2ZQS73acdFqojKi8IKSy LxdDEVovNDx28hfI2hu/CIv1owYteHUesOcFV1P/GPZOgok7gfoIg8Gr7UNROk2Ke/GF R8mw== X-Gm-Message-State: AGRZ1gIAJnuW8Dl2iuMBsowYTPn0pvdeTSUJKzBrZUl1KXeb77nyMK66 9qpa/WpScwQ2x/51pp2uup4= X-Google-Smtp-Source: AFSGD/Xio1TvGPgy2os+AZRLNbtVtIw/lhXyOVFepfQYfJzDyYa1STZR0ASTvzzhMWxjhdQznxR7dg== X-Received: by 2002:a1c:d0cd:: with SMTP id h196mr6052057wmg.13.1542616738617; Mon, 19 Nov 2018 00:38:58 -0800 (PST) Received: from gmail.com (2E8B0CD5.catv.pool.telekom.hu. [46.139.12.213]) by smtp.gmail.com with ESMTPSA id x194-v6sm88903564wmd.41.2018.11.19.00.38.57 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 19 Nov 2018 00:38:57 -0800 (PST) Date: Mon, 19 Nov 2018 09:38:55 +0100 From: Ingo Molnar To: Linus Torvalds Cc: Jiri Kosina , Thomas Gleixner , Peter Zijlstra , Josh Poimboeuf , Andrea Arcangeli , David Woodhouse , Andi Kleen , Tim Chen , Casey Schaufler , Linux List Kernel Mailing , the arch/x86 maintainers , stable@vger.kernel.org Subject: Re: STIBP by default.. Revert? Message-ID: <20181119083855.GA129733@gmail.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Linus Torvalds wrote: > This was marked for stable, and honestly, nowhere in the discussion > did I see any mention of just *how* bad the performance impact of this > was. Yeah. This was an oversight - we'll fix it! > When performance goes down by 50% on some loads, people need to start > asking themselves whether it was worth it. It's apparently better to > just disable SMT entirely, which is what security-conscious people do > anyway. > > So why do that STIBP slow-down by default when the people who *really* > care already disabled SMT? > > I think we should use the same logic as for L1TF: we default to > something that doesn't kill performance. Warn once about it, and let > the crazy people say "I'd rather take a 50% performance hit than > worry about a theoretical issue". Yeah, absolutely. We'll also require performance measurements in changelogs enabling any sort of mitigation feature from now on - this requirement was implicit but 53c613fe6349 flew in under the radar, so it's going to be explicit an explicit requirement. Thanks, Ingo