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 16F96C606C2 for ; Mon, 8 Jul 2019 15:11:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EE92C216C4 for ; Mon, 8 Jul 2019 15:11:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732176AbfGHPLp (ORCPT ); Mon, 8 Jul 2019 11:11:45 -0400 Received: from mga09.intel.com ([134.134.136.24]:58570 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732162AbfGHPLm (ORCPT ); Mon, 8 Jul 2019 11:11:42 -0400 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by orsmga102.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 08 Jul 2019 08:11:41 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.63,466,1557212400"; d="scan'208";a="165482391" Received: from tassilo.jf.intel.com (HELO tassilo.localdomain) ([10.7.201.137]) by fmsmga008.fm.intel.com with ESMTP; 08 Jul 2019 08:11:40 -0700 Received: by tassilo.localdomain (Postfix, from userid 1000) id 3F381301BB6; Mon, 8 Jul 2019 08:11:41 -0700 (PDT) Date: Mon, 8 Jul 2019 08:11:41 -0700 From: Andi Kleen To: Peter Zijlstra Cc: Wei Wang , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, pbonzini@redhat.com, kan.liang@intel.com, mingo@redhat.com, rkrcmar@redhat.com, like.xu@intel.com, jannh@google.com, arei.gonglei@huawei.com, jmattson@google.com Subject: Re: [PATCH v7 10/12] KVM/x86/lbr: lazy save the guest lbr stack Message-ID: <20190708151141.GL31027@tassilo.jf.intel.com> References: <1562548999-37095-1-git-send-email-wei.w.wang@intel.com> <1562548999-37095-11-git-send-email-wei.w.wang@intel.com> <20190708145326.GO3402@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190708145326.GO3402@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.11.3 (2019-02-01) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org > I don't understand a word of that. > > Who cares if the LBR MSRs are touched; the guest expects up-to-date > values when it does reads them. This is for only when the LBRs are disabled in the guest. It doesn't make sense to constantly save/restore disabled LBRs, which would be a large overhead for guests that don't use LBRs. When the LBRs are enabled they always need to be saved/restored as you say. -Andi