From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752215AbdBMT2m (ORCPT <rfc822;w@1wt.eu>);
        Mon, 13 Feb 2017 14:28:42 -0500
Received: from Galois.linutronix.de ([146.0.238.70]:41089 "EHLO
        Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751313AbdBMT2l (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Mon, 13 Feb 2017 14:28:41 -0500
Date: Mon, 13 Feb 2017 20:28:21 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Andy Lutomirski <luto@amacapital.net>
cc: Dexuan Cui <decui@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        "x86@kernel.org" <x86@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>
Subject: Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read
 method
In-Reply-To: <CALCETrX3UkxNkAtyL+fk=HEtBRmd3GCkw+qLur4aVgp7rn4WCA@mail.gmail.com>
Message-ID: <alpine.DEB.2.20.1702132027300.3790@nanos>
References: <20170209141052.18694-1-vkuznets@redhat.com> <20170209141052.18694-3-vkuznets@redhat.com> <alpine.DEB.2.20.1702091806400.3604@nanos> <BLUPR0301MB20986D17696C3E658D511FACCC450@BLUPR0301MB2098.namprd03.prod.outlook.com> <87d1eqqlkv.fsf@vitty.brq.redhat.com>
 <alpine.DEB.2.20.1702101326520.4036@nanos> <BLUPR0301MB2098E1283D42BD2C3772D839CC440@BLUPR0301MB2098.namprd03.prod.outlook.com> <alpine.DEB.2.20.1702101852000.4036@nanos> <MWHPR03MB266952220A7E1FA0F2D38453BF590@MWHPR03MB2669.namprd03.prod.outlook.com>
 <CALCETrX3UkxNkAtyL+fk=HEtBRmd3GCkw+qLur4aVgp7rn4WCA@mail.gmail.com>
User-Agent: Alpine 2.20 (DEB 67 2015-01-07)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, 13 Feb 2017, Andy Lutomirski wrote:

> On Sun, Feb 12, 2017 at 11:49 PM, Dexuan Cui <decui@microsoft.com> wrote:
> >> From: Thomas Gleixner [mailto:tglx@linutronix.de]
> >> Sent: Saturday, February 11, 2017 02:02
> >>  ...
> >> That's important if the stuff happens cross CPU. If the update happens on
> >> the same CPU then this is a different story and as there are VMexits
> >> involved they might provide the required ordering already. But I can't tell
> >> as I have no idea how that host side thing is done.
> >>
> >>       tglx
> >
> > IMO Hyper-V TSC page clocksource here seems pretty similar to KVM's pvclock,
> > So I would guess "the structure is only updated just before reentering the guest
> > after some VM event" (https://rwmj.wordpress.com/2010/10/15/kvm-pvclock/),
> > that is, the update should happen on the same CPU, I guess.
> 
> If the patch is correct, there is one of these shared by all vCPUs, so
> this is not a sufficient explanation.

Right, because that's ony ONE TSC page for the whole guest and then the seq
stuff really lacks barriers.

Thanks,

	tglx