From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f197.google.com (mail-pf0-f197.google.com [209.85.192.197]) by kanga.kvack.org (Postfix) with ESMTP id 674DF6B0012 for ; Fri, 23 Mar 2018 20:41:00 -0400 (EDT) Received: by mail-pf0-f197.google.com with SMTP id p9so7534134pfk.5 for ; Fri, 23 Mar 2018 17:41:00 -0700 (PDT) Received: from mga11.intel.com (mga11.intel.com. [192.55.52.93]) by mx.google.com with ESMTPS id t6-v6si9255178plr.503.2018.03.23.17.40.59 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 23 Mar 2018 17:40:59 -0700 (PDT) Subject: Re: [PATCH 00/11] Use global pages with PTI References: <20180323174447.55F35636@viggo.jf.intel.com> From: Dave Hansen Message-ID: Date: Fri, 23 Mar 2018 17:40:57 -0700 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Linus Torvalds Cc: Linux Kernel Mailing List , linux-mm , Andrea Arcangeli , Andrew Lutomirski , Kees Cook , Hugh Dickins , =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= , the arch/x86 maintainers , namit@vmware.com On 03/23/2018 11:26 AM, Linus Torvalds wrote: > On Fri, Mar 23, 2018 at 10:44 AM, Dave Hansen > wrote: >> >> This adds one major change from the last version of the patch set >> (present in the last patch). It makes all kernel text global for non- >> PCID systems. This keeps kernel data protected always, but means that >> it will be easier to find kernel gadgets via meltdown on old systems >> without PCIDs. This heuristic is, I think, a reasonable one and it >> keeps us from having to create any new pti=foo options > > Sounds sane. > > The patches look reasonable, but I hate seeing a patch series like > this where the only ostensible reason is performance, and there are no > performance numbers anywhere.. Well, rats. This somehow makes things slower with PCIDs on. I thought I reversed the numbers, but I actually do a "grep -c GLB /sys/kernel/debug/page_tables/kernel" and record that in my logs right next to the output of time(1), so it's awfully hard to screw up. This is time doing a modestly-sized kernel compile on a 4-core Skylake desktop. User Time Kernel Time Clock Elapsed Baseline ( 0 GLB PTEs) 803.79 67.77 237.30 w/series (28 GLB PTEs) 807.70 (+0.7%) 68.07 (+0.7%) 238.07 (+0.3%) Without PCIDs, it behaves the way I would expect. I'll ask around, but I'm open to any ideas about what the heck might be causing this.