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=-5.3 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 24BBFC2D0E2 for ; Tue, 22 Sep 2020 14:50:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DDD27208A9 for ; Tue, 22 Sep 2020 14:50:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="eEKGFUCL" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726662AbgIVOuu (ORCPT ); Tue, 22 Sep 2020 10:50:50 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:51024 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726566AbgIVOuu (ORCPT ); Tue, 22 Sep 2020 10:50:50 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1600786248; 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=Dum7Y2sVYostqgHyuDYV4BcH6A2VsvfCX37jDJ02YsQ=; b=eEKGFUCLKDIh40mSb4NT9dbr1826zDs1IiGYQf3nC/E2D1IgezfYr3jSg079/O7COcEeXD ah/NvZ52IO+kS32iG/4RBjloU0gy2xLSWjqrZacNZbE24FOt3Q07DWzvraXz9zt9T3dDLa 0Gu9763bfcAfBNz+EcvPkf6eWlN/GSs= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-401-71DJAb7KOSul45HHYVye5w-1; Tue, 22 Sep 2020 10:50:45 -0400 X-MC-Unique: 71DJAb7KOSul45HHYVye5w-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id EEE06807341; Tue, 22 Sep 2020 14:50:42 +0000 (UTC) Received: from starship (unknown [10.35.206.154]) by smtp.corp.redhat.com (Postfix) with ESMTP id B7D4E78822; Tue, 22 Sep 2020 14:50:37 +0000 (UTC) Message-ID: <7db1383cc9d40f76a02076c3b86cf832fd7463cc.camel@redhat.com> Subject: Re: [PATCH v2 1/1] KVM: x86: fix MSR_IA32_TSC read for nested migration From: Maxim Levitsky To: Paolo Bonzini , Sean Christopherson Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Jim Mattson , Wanpeng Li , Ingo Molnar , Borislav Petkov , Thomas Gleixner , Vitaly Kuznetsov , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Joerg Roedel , "H. Peter Anvin" Date: Tue, 22 Sep 2020 17:50:36 +0300 In-Reply-To: References: <20200921103805.9102-1-mlevitsk@redhat.com> <20200921103805.9102-2-mlevitsk@redhat.com> <20200921162326.GB23989@linux.intel.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.3 (3.36.3-1.fc32) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2020-09-22 at 14:50 +0200, Paolo Bonzini wrote: > On 21/09/20 18:23, Sean Christopherson wrote: > > Avoid "should" in code comments and describe what the code is doing, not what > > it should be doing. The only exception for this is when the code has a known > > flaw/gap, e.g. "KVM should do X, but because of Y, KVM actually does Z". > > > > > + * return it's real L1 value so that its restore will be correct. > > s/it's/its > > > > Perhaps add "unconditionally" somewhere, since arch.tsc_offset can also contain > > the L1 value. E.g. > > > > * Unconditionally return L1's TSC offset on userspace reads > > * so that userspace reads and writes always operate on L1's > > * offset, e.g. to ensure deterministic behavior for migration. > > */ > > > > Technically the host need not restore MSR_IA32_TSC at all. This follows > the idea of the discussion with Oliver Upton about transmitting the > state of the kvmclock heuristics to userspace, which include a (TSC, > CLOCK_MONOTONIC) pair to transmit the offset to the destination. All > that needs to be an L1 value is then the TSC value in that pair. > > I'm a bit torn over this patch. On one hand it's an easy solution, on > the other hand it's... just wrong if KVM_GET_MSR is used for e.g. > debugging the guest. Could you explain why though? After my patch, the KVM_GET_MSR will consistently read the L1 TSC, just like all other MSRs as I explained. I guess for debugging, this should work? The fact that TSC reads with the guest offset is a nice exception made for the guests, that insist on reading this msr without inteception and not using rdtsc. Best regards, Maxim Levitsky > > I'll talk to Maxim and see if he can work on the kvmclock migration stuff. > > Paolo >