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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 615B5C3A59F for ; Thu, 29 Aug 2019 14:30:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 39BCA23405 for ; Thu, 29 Aug 2019 14:30:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727257AbfH2Oa5 (ORCPT ); Thu, 29 Aug 2019 10:30:57 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50544 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726852AbfH2Oa4 (ORCPT ); Thu, 29 Aug 2019 10:30:56 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 4BE413D966; Thu, 29 Aug 2019 14:30:56 +0000 (UTC) Received: from pauld.bos.csb (dhcp-17-51.bos.redhat.com [10.18.17.51]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0330660461; Thu, 29 Aug 2019 14:30:52 +0000 (UTC) Date: Thu, 29 Aug 2019 10:30:51 -0400 From: Phil Auld To: Peter Zijlstra Cc: Matthew Garrett , Vineeth Remanan Pillai , Nishanth Aravamudan , Julien Desfossez , Tim Chen , mingo@kernel.org, tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, subhra.mazumdar@oracle.com, fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com, Aaron Lu , Aubrey Li , Valentin Schneider , Mel Gorman , Pawan Gupta , Paolo Bonzini Subject: Re: [RFC PATCH v3 00/16] Core scheduling v3 Message-ID: <20190829143050.GA7262@pauld.bos.csb> References: <20190827211417.snpwgnhsu5t6u52y@srcf.ucam.org> <20190827215035.GH2332@hirez.programming.kicks-ass.net> <20190828153033.GA15512@pauld.bos.csb> <20190828160114.GE17205@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190828160114.GE17205@worktop.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Thu, 29 Aug 2019 14:30:56 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 28, 2019 at 06:01:14PM +0200 Peter Zijlstra wrote: > On Wed, Aug 28, 2019 at 11:30:34AM -0400, Phil Auld wrote: > > On Tue, Aug 27, 2019 at 11:50:35PM +0200 Peter Zijlstra wrote: > > > > And given MDS, I'm still not entirely convinced it all makes sense. If > > > it were just L1TF, then yes, but now... > > > > I was thinking MDS is really the reason for this. L1TF has mitigations but > > the only current mitigation for MDS for smt is ... nosmt. > > L1TF has no known mitigation that is SMT safe. The moment you have > something in your L1, the other sibling can read it using L1TF. > > The nice thing about L1TF is that only (malicious) guests can exploit > it, and therefore the synchronizatin context is VMM. And it so happens > that VMEXITs are 'rare' (and already expensive and thus lots of effort > has already gone into avoiding them). > > If you don't use VMs, you're good and SMT is not a problem. > > If you do use VMs (and do/can not trust them), _then_ you need > core-scheduling; and in that case, the implementation under discussion > misses things like synchronization on VMEXITs due to interrupts and > things like that. > > But under the assumption that VMs don't generate high scheduling rates, > it can work. > > > The current core scheduler implementation, I believe, still has (theoretical?) > > holes involving interrupts, once/if those are closed it may be even less > > attractive. > > No; so MDS leaks anything the other sibling (currently) does, this makes > _any_ privilidge boundary a synchronization context. > > Worse still, the exploit doesn't require a VM at all, any other task can > get to it. > > That means you get to sync the siblings on lovely things like system > call entry and exit, along with VMM and anything else that one would > consider a privilidge boundary. Now, system calls are not rare, they > are really quite common in fact. Trying to sync up siblings at the rate > of system calls is utter madness. > > So under MDS, SMT is completely hosed. If you use VMs exclusively, then > it _might_ work because a 'pure' host doesn't schedule that often > (maybe, same assumption as for L1TF). > > Now, there have been proposals of moving the privilidge boundary further > into the kernel. Just like PTI exposes the entry stack and code to > Meltdown, the thinking is, lets expose more. By moving the priv boundary > the hope is that we can do lots of common system calls without having to > sync up -- lots of details are 'pending'. Thanks for clarifying. My understanding is (somewhat) less fuzzy now. :) I think, though, that you were basically agreeing with me that the current core scheduler does not close the holes, or am I reading that wrong. Cheers, Phil --