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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9EBABC433EF for ; Wed, 1 Jun 2022 08:25:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1350707AbiFAIZa (ORCPT ); Wed, 1 Jun 2022 04:25:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39162 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1350773AbiFAIZJ (ORCPT ); Wed, 1 Jun 2022 04:25:09 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 68A995F58 for ; Wed, 1 Jun 2022 01:25:08 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1654071907; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=9PuqWSM1RqEiAAMMqRkf0RREiczy16fCzdCz4SZ9Yh8=; b=a3X3DM8Q450FYyesFUrHPF2J/QSKCH5+ZUisxb+Kbv+QX7wyMo398Ra6iaImtmW0AacKrV XM+xLz9Ds80MpETIpazRnyC8VSLUxSizGfbQMVStbTVa0VvutwAV2mqZyC6YxRV24huX4s yhrHFJnR8/d5YiAgA1IUVZvwC+Umh6I= Received: from mail-wm1-f70.google.com (mail-wm1-f70.google.com [209.85.128.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-50-gQ_nIOOEPZ6Yd-dx0n06rA-1; Wed, 01 Jun 2022 04:25:06 -0400 X-MC-Unique: gQ_nIOOEPZ6Yd-dx0n06rA-1 Received: by mail-wm1-f70.google.com with SMTP id k5-20020a05600c1c8500b003974c5d636dso783845wms.1 for ; Wed, 01 Jun 2022 01:25:06 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:subject:from:in-reply-to:date:cc :content-transfer-encoding:message-id:references:to; bh=9PuqWSM1RqEiAAMMqRkf0RREiczy16fCzdCz4SZ9Yh8=; b=RCmPJIoUd4MpgKUqVvrsWapVe3VQF6AQ/Jj/nMQH/eeER1JNp35S9dHJTM3/cW0fQx 8aWOkJw2JVC82KMI+Vo9ktXpebaf1PUOLbtrD1S5VXQgNaapBoPnsv2+zwsn8cfBqc1g WBhYK+89ZchmT51Lsc6qsudpSo2YIKMm2U/cqLGb1oO37eLgwnCtZM6ND2Hx0OjSBqf3 eBUfCccRES99SZqT7shksxz3ZGAyOoZvLOx378MIGtIIqfnlmjE7jQk1YjtYcp30sxah zsAryBrrM/hjt5LjD80LJ7DS6vP5cx7U/lTZH3MTOAxQFhAUMIwpt8s3Yxa74dV4BYem sqqQ== X-Gm-Message-State: AOAM531VOAaUO2SZYbdq1PYzvlmND2l2dGeiF3V4OVV5ceOECoZ/5UuH IBE/fP0xjbaFYpiVof2a81eo9obAKqJOp/WzV63G2dAcKcK3kYM2cc9R32jPAxtrQFxcMxDQayV wpH4AWa12Oodchgdf2T0bZRVg X-Received: by 2002:adf:fb46:0:b0:210:2316:dd02 with SMTP id c6-20020adffb46000000b002102316dd02mr17673880wrs.557.1654071905005; Wed, 01 Jun 2022 01:25:05 -0700 (PDT) X-Google-Smtp-Source: ABdhPJyRD/IRDpVLqPBmdGC0pIb4xFXUChVcdXWE70cLKfJv6Nh+TFUtXKaRUjy4B0vznzkaEH6ugw== X-Received: by 2002:adf:fb46:0:b0:210:2316:dd02 with SMTP id c6-20020adffb46000000b002102316dd02mr17673852wrs.557.1654071904613; Wed, 01 Jun 2022 01:25:04 -0700 (PDT) Received: from smtpclient.apple ([2a01:e0a:834:5aa0:2c2a:4832:6517:63a]) by smtp.gmail.com with ESMTPSA id p15-20020a05600c358f00b003973ea7e725sm6337978wmq.0.2022.06.01.01.25.03 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Wed, 01 Jun 2022 01:25:04 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3696.80.82.1.1\)) Subject: Re: ...\n From: Christophe de Dinechin In-Reply-To: <87r148olol.fsf@redhat.com> Date: Wed, 1 Jun 2022 10:25:01 +0200 Cc: Peter Zijlstra , "Durrant, Paul" , "Allister, Jack" , "bp@alien8.de" , "diapop@amazon.co.uk" , "hpa@zytor.com" , "jmattson@google.com" , "joro@8bytes.org" , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "metikaya@amazon.co.uk" , "mingo@redhat.com" , "pbonzini@redhat.com" , "rkrcmar@redhat.com" , "sean.j.christopherson@intel.com" , "tglx@linutronix.de" , "wanpengli@tencent.com" , "x86@kernel.org" Content-Transfer-Encoding: quoted-printable Message-Id: References: <20220531140236.1435-1-jalliste@amazon.com> <059ab3327ac440479ecfdf49fa054347@EX13D32EUC003.ant.amazon.com> <87r148olol.fsf@redhat.com> To: Vitaly Kuznetsov X-Mailer: Apple Mail (2.3696.80.82.1.1) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On 1 Jun 2022, at 10:03, Vitaly Kuznetsov wrote: >=20 > Peter Zijlstra writes: >=20 >> On Tue, May 31, 2022 at 02:52:04PM +0000, Durrant, Paul wrote: >=20 > ... >=20 >>>=20 >>> I'll bite... What's ludicrous about wanting to run a guest at a = lower >>> CPU freq to minimize observable change in whatever workload it is >>> running? >>=20 >> *why* would you want to do that? Everybody wants their stuff done >> faster. >>=20 >=20 > FWIW, I can see a valid use-case: imagine you're running some software > which calibrates itself in the beginning to run at some desired real > time speed but then the VM running it has to be migrated to a host = with > faster (newer) CPUs. I don't have a real world examples out of top of = my > head but I remember some old DOS era games were impossible to play on > newer CPUs because everything was happenning too fast. Maybe that's = the > case :-) The PC version of Alpha Waves was such an example, but Frederick Raynal, who did the port, said it was the last time he made the mistake. That = was 1990 :-) More seriously, what about mitigating timing-based remote attacks by arbitrarily changing the CPU frequency and injecting noise in the = timing? That could be a valid use case, no? Although I can think of about a million other ways of doing this more efficiently=E2=80=A6 >=20 > --=20 > Vitaly >=20