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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 252D7C31E46 for ; Wed, 12 Jun 2019 14:59:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EF0AB20866 for ; Wed, 12 Jun 2019 14:59:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2439937AbfFLO7Q (ORCPT ); Wed, 12 Jun 2019 10:59:16 -0400 Received: from mx2.suse.de ([195.135.220.15]:51758 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S2437985AbfFLO7Q (ORCPT ); Wed, 12 Jun 2019 10:59:16 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id C42B2AEEE; Wed, 12 Jun 2019 14:59:14 +0000 (UTC) Subject: Re: [PATCH] x86/xen: disable nosmt in Xen guests To: Peter Zijlstra Cc: xen-devel@lists.xenproject.org, x86@kernel.org, linux-kernel@vger.kernel.org, Boris Ostrovsky , Stefano Stabellini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" References: <20190612101228.23898-1-jgross@suse.com> <20190612114836.GI3436@hirez.programming.kicks-ass.net> From: Juergen Gross Message-ID: <74c04cc4-4b05-2bca-d788-ea3605853fcc@suse.com> Date: Wed, 12 Jun 2019 16:59:12 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190612114836.GI3436@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: de-DE Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12.06.19 13:48, Peter Zijlstra wrote: > On Wed, Jun 12, 2019 at 12:12:28PM +0200, Juergen Gross wrote: >> When running as a Xen guest selecting "nosmt" either via command line >> or implicitly via default settings makes no sense, as the guest has no >> clue about the real system topology it is running on. With Xen it is >> the hypervisor's job to ensure the proper bug mitigations are active >> regarding smt settings. >> >> So when running as a Xen guest set cpu_smt_control to "not supported" >> in order to avoid disabling random vcpus. > > If it doesn't make sense; then the topology should not expose SMT > threads and the knob will not have any effect. Yes, that's the theory. I agree completely, but this is ongoing work which will need some more time. It probably would have been ready for some time now, but some recent processor bugs required a shift in priorities what to do first. In order to run a new kernel on existing Xen we need that patch to avoid disabling random cpus. Juergen