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 26461C31E46 for ; Wed, 12 Jun 2019 10:36:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ED699208CB for ; Wed, 12 Jun 2019 10:36:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2408878AbfFLKgx (ORCPT ); Wed, 12 Jun 2019 06:36:53 -0400 Received: from mx2.suse.de ([195.135.220.15]:43978 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S2408253AbfFLKgv (ORCPT ); Wed, 12 Jun 2019 06:36:51 -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 A0633AF97; Wed, 12 Jun 2019 10:36:50 +0000 (UTC) Subject: Re: [Xen-devel] [PATCH] x86/xen: disable nosmt in Xen guests To: Jan Beulich Cc: Borislav Petkov , Stefano Stabellini , the arch/x86 maintainers , tglx@linutronix.de, xen-devel , Boris Ostrovsky , mingo@redhat.com, linux-kernel@vger.kernel.org, hpa@zytor.com References: <20190612101228.23898-1-jgross@suse.com> <5D00D1A602000078002376A9@prv1-mh.provo.novell.com> From: Juergen Gross Message-ID: <8392fdc4-a6b2-a3aa-dca6-0a0ad7a411be@suse.com> Date: Wed, 12 Jun 2019 12:36:49 +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: <5D00D1A602000078002376A9@prv1-mh.provo.novell.com> 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 12:19, Jan Beulich wrote: >>>> On 12.06.19 at 12:12, 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. > > I don't agree with the second sentence: It is in principle fine for the > hypervisor to expose HT (i.e. not disable it as bug mitigation), and > leave it to the guest kernels to protect themselves. We're just not > at the point yet where Xen offers sufficient / reliable data to guest > kernels to do so, so _for the time being_ what you say is correct. Okay, I'll add something like: This is true as long Xen doesn't support core scheduling together with exposing the (then) correct sibling information to the guest and indicating that case via a sutable interface. Juergen