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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 E0004C433DF for ; Thu, 21 May 2020 06:44:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AA9A3207FB for ; Thu, 21 May 2020 06:44:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728235AbgEUGo3 (ORCPT ); Thu, 21 May 2020 02:44:29 -0400 Received: from mga12.intel.com ([192.55.52.136]:23133 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726506AbgEUGo3 (ORCPT ); Thu, 21 May 2020 02:44:29 -0400 IronPort-SDR: gRM+zXu+4i+vWI3un+JixCaJ2pQ5RH4ZmUH7SDNZ76+AMqGHuiQNAupU/xFl3ejSEeYvWNWMZ5 iCl1J5MS1S1A== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 20 May 2020 23:44:27 -0700 IronPort-SDR: 1PYEqXn0JHyXkDSXirO35ZdfQy7Cce8666rbCprvjYMq/DXcm6jYSWAd+sq0ncUljzcGoFGZHO ljEPBTXrbGTA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,416,1583222400"; d="scan'208";a="289620690" Received: from txu2-mobl.ccr.corp.intel.com (HELO [10.238.4.118]) ([10.238.4.118]) by fmsmga004.fm.intel.com with ESMTP; 20 May 2020 23:44:26 -0700 Subject: Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT_CONTROL unconditionally To: Xiaoyao Li Cc: Paolo Bonzini , Maxim Levitsky , kvm@vger.kernel.org, linux-kernel@vger.kernel.org References: <20200520160740.6144-1-mlevitsk@redhat.com> <20200520160740.6144-3-mlevitsk@redhat.com> <81228a0e-7797-4f34-3d6d-5b0550c10a8f@intel.com> From: Tao Xu Message-ID: <6c99b807-fe67-23b5-3332-b7200bf5d639@intel.com> Date: Thu, 21 May 2020 14:44:25 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/21/2020 2:37 PM, Xiaoyao Li wrote: > On 5/21/2020 1:28 PM, Tao Xu wrote: >> >> >> On 5/21/2020 12:33 PM, Xiaoyao Li wrote: >>> On 5/21/2020 5:05 AM, Paolo Bonzini wrote: >>>> On 20/05/20 18:07, Maxim Levitsky wrote: >>>>> This msr is only available when the host supports WAITPKG feature. >>>>> >>>>> This breaks a nested guest, if the L1 hypervisor is set to ignore >>>>> unknown msrs, because the only other safety check that the >>>>> kernel does is that it attempts to read the msr and >>>>> rejects it if it gets an exception. >>>>> >>>>> Fixes: 6e3ba4abce KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL >>>>> >>>>> Signed-off-by: Maxim Levitsky >>>>> --- >>>>>   arch/x86/kvm/x86.c | 4 ++++ >>>>>   1 file changed, 4 insertions(+) >>>>> >>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>>>> index fe3a24fd6b263..9c507b32b1b77 100644 >>>>> --- a/arch/x86/kvm/x86.c >>>>> +++ b/arch/x86/kvm/x86.c >>>>> @@ -5314,6 +5314,10 @@ static void kvm_init_msr_list(void) >>>>>               if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_EVENTSEL0 >= >>>>>                   min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp)) >>>>>                   continue; >>>>> +            break; >>>>> +        case MSR_IA32_UMWAIT_CONTROL: >>>>> +            if (!kvm_cpu_cap_has(X86_FEATURE_WAITPKG)) >>>>> +                continue; >>>>>           default: >>>>>               break; >>>>>           } >>>> >>>> The patch is correct, and matches what is done for the other entries of >>>> msrs_to_save_all.  However, while looking at it I noticed that >>>> X86_FEATURE_WAITPKG is actually never added, and that is because it was >>>> also not added to the supported CPUID in commit e69e72faa3a0 ("KVM: >>>> x86: >>>> Add support for user wait instructions", 2019-09-24), which was before >>>> the kvm_cpu_cap mechanism was added. >>>> >>>> So while at it you should also fix that.  The right way to do that >>>> is to >>>> add a >>>> >>>>          if (vmx_waitpkg_supported()) >>>>                  kvm_cpu_cap_check_and_set(X86_FEATURE_WAITPKG); >>> >>> + Tao >>> >>> I remember there is certainly some reason why we don't expose WAITPKG >>> to guest by default. >>> >>> Tao, please help clarify it. >>> >>> Thanks, >>> -Xiaoyao >>> >> >> Because in VM, umwait and tpause can put a (psysical) CPU into a power >> saving state. So from host view, this cpu will be 100% usage by VM. >> Although umwait and tpause just cause short wait(maybe 100 >> microseconds), we still want to unconditionally expose WAITPKG in VM. > > I guess you typed "unconditionally" by mistake that you meant to say > "conditionally" in fact? I am sorry, I mean: By default, we don't expose WAITPKG to guest. For QEMU, we can use "-overcommit cpu-pm=on" to use WAITPKG.