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=-7.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 96B7FC07E95 for ; Wed, 7 Jul 2021 15:08:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7C5A861CC2 for ; Wed, 7 Jul 2021 15:08:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232017AbhGGPLd (ORCPT ); Wed, 7 Jul 2021 11:11:33 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:28850 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232231AbhGGPLd (ORCPT ); Wed, 7 Jul 2021 11:11:33 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1625670532; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=aWcVKjuywp9lh2ZpwGA5QOnKrFtu8NroiBWGr7IsBn8=; b=a28QXZwcYX5I/YLrg1MDdlWEUmXUAtcH5JBcOlEOkiMVUh4HpNTPvV7Wg+uS2Bhl8+3nSB Dh5xkYvITzmzLH1LYuf5SRofuSYNiBhEbogm+Bxpoq3vn6m8RhnNNvQyQa8dwLw0yH9rEt LUOl5TIEj9wh1GGcbtPHiEMLIJgjM1E= Received: from mail-lj1-f198.google.com (mail-lj1-f198.google.com [209.85.208.198]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-294-tg-T8N9MOdmlYl65Glu3Ww-1; Wed, 07 Jul 2021 11:08:51 -0400 X-MC-Unique: tg-T8N9MOdmlYl65Glu3Ww-1 Received: by mail-lj1-f198.google.com with SMTP id q13-20020a2e914d0000b02901752c889bcdso643905ljg.20 for ; Wed, 07 Jul 2021 08:08:51 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=aWcVKjuywp9lh2ZpwGA5QOnKrFtu8NroiBWGr7IsBn8=; b=ZaC4gZadC447Dnnnt8HKQfpyg8E4c/48u4mBiLErYK7uOc5GA45bMddUryRMIHjqe7 VWpNYh0p8p0nQ7Dh1b9t4EAG/iBCc0773E/b3x4pfNuR+ZZdUjDLUIA2qFD6f91ZGRRn uSrLQtsvobn5YHhq2fG5OQonYVewldgNYEo8QfoTpU1lPCQhNa/ZHy2U5ivzezSHMEly cnaovST8WBy2yqEQV5jcBYtnQRuCaFIib5Yo1uv8PER2wxXZ6i1hG2t4NEDVJxv/mQzO BzH+cCDKeWopPSc1WhxwvEo/on4ekm2M9A+xWEGriN/jN4EwUzOeJyPlPO1AQNxm3VT0 wrMw== X-Gm-Message-State: AOAM530iPDPFVd2UDZWfn5jz2FZGdAeZTLXgQnSSviOw/ikdk5hvkl5T sC1A2LE3lYl983r2QcLI4dzq4wZ3AKxVgtPsj+hjQTuc2K8zkI5uzhzW0a7wSyfySeHz/eYZVgQ f8hhl6d4CrcJn1WP8zpJDWVI1eOEkRVhXUFY59OEJrXxZ/A== X-Received: by 2002:a19:2d0f:: with SMTP id k15mr20212365lfj.237.1625670529919; Wed, 07 Jul 2021 08:08:49 -0700 (PDT) X-Google-Smtp-Source: ABdhPJxlXVR/XQy7YZ+nddw8d1d1hF4je9/71Rlb3CsaZQ2XC6f0NCZVAnPc1T3/8u2rsmmgs33oME3cBU/Gin1VvQg= X-Received: by 2002:a19:2d0f:: with SMTP id k15mr20212330lfj.237.1625670529596; Wed, 07 Jul 2021 08:08:49 -0700 (PDT) MIME-Version: 1.0 References: <20210706195233.h6w4cm73oktfqpgz@habkost.net> <4cc2c5fe-2153-05c5-dedd-8cb650753740@redhat.com> <671be35f-220a-f583-aa31-3a2da7dae93a@redhat.com> In-Reply-To: <671be35f-220a-f583-aa31-3a2da7dae93a@redhat.com> From: Eduardo Habkost Date: Wed, 7 Jul 2021 11:08:32 -0400 Message-ID: Subject: Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE To: Paolo Bonzini Cc: Pawan Gupta , Thomas Gleixner , Borislav Petkov , Jonathan Corbet , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , x86@kernel.org, "H. Peter Anvin" , "Paul E. McKenney" , Randy Dunlap , Andrew Morton , "Maciej W. Rozycki" , Viresh Kumar , Vlastimil Babka , Tony Luck , Sean Christopherson , Kyung Min Park , Fenghua Yu , Ricardo Neri , Tom Lendacky , Juergen Gross , Krish Sadhukhan , Kan Liang , Joerg Roedel , Victor Ding , Srinivas Pandruvada , Brijesh Singh , Dave Hansen , Mike Rapoport , Anthony Steinhauser , Anand K Mistry , Andi Kleen , Miguel Ojeda , Nick Desaulniers , Joe Perches , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, kvm@vger.kernel.org, Jiri Denemark , "libvir-list@redhat.com" , Michal Privoznik Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org CCing libvir-list, Jiri Denemark, Michal Privoznik, so they are aware that the definition of "supported CPU features" will probably become a bit more complex in the future. On Tue, Jul 6, 2021 at 5:58 PM Paolo Bonzini wrote: > > On 06/07/21 23:33, Eduardo Habkost wrote: > > On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini wrote: > >> It's a bit tricky, because HLE and RTM won't really behave well. An old > >> guest that sees RTM=1 might end up retrying and aborting transactions > >> too much. So I'm not sure that a QEMU "-cpu host" guest should have HLE > >> and RTM enabled. > > > > Is the purpose of GET_SUPPORTED_CPUID to return what is supported by > > KVM, or to return what "-cpu host" should enable by default? They are > > conflicting requirements in this case. > > In theory there is GET_EMULATED_CPUID for the former, so it should be > the latter. In practice neither QEMU nor Libvirt use it; maybe now we > have a good reason to add it, but note that userspace could also check > host RTM_ALWAYS_ABORT. > > > Returning HLE=1,RTM=1 in GET_SUPPORTED_CPUID makes existing userspace > > take bad decisions until it's updated. > > > > Returning HLE=0,RTM=0 in GET_SUPPORTED_CPUID prevents existing > > userspace from resuming existing VMs (despite being technically > > possible). > > > > The first option has an easy workaround that doesn't require a > > software update (disabling HLE/RTM in the VM configuration). The > > second option doesn't have a workaround. I'm inclined towards the > > first option. > > The default has already been tsx=off for a while though, so checking > either GET_EMULATED_CPUID or host RTM_ALWAYS_ABORT in userspace might > also be feasible for those that are still on tsx=on. This sounds like a perfect use case for GET_EMULATED_CPUID. My only concern is breaking existing userspace. But if this was already broken for a few kernel releases due to tsx=off being the default, maybe GET_EMULATED_CPUID will be a reasonable approach. -- Eduardo