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 S934386AbdBQRzM (ORCPT <rfc822;w@1wt.eu>);
        Fri, 17 Feb 2017 12:55:12 -0500
Received: from Galois.linutronix.de ([146.0.238.70]:58083 "EHLO
        Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S934096AbdBQRzL (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 17 Feb 2017 12:55:11 -0500
Date: Fri, 17 Feb 2017 18:55:02 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Andy Lutomirski <luto@amacapital.net>
cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>, X86 ML <x86@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Dexuan Cui <decui@microsoft.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        devel@linuxdriverproject.org,
        Linux Virtualization <virtualization@lists.linux-foundation.org>
Subject: Re: [PATCH v2 0/3] x86/vdso: Add Hyper-V TSC page clocksource
 support
In-Reply-To: <CALCETrXB8CufQujLAg6bbq=DGAMUE293CF7L4Kp+mCSoNWyuBg@mail.gmail.com>
Message-ID: <alpine.DEB.2.20.1702171853480.3536@nanos>
References: <20170214124408.25931-1-vkuznets@redhat.com> <alpine.DEB.2.20.1702141552360.3635@nanos> <87a89o22ml.fsf@vitty.brq.redhat.com> <CALCETrUNdemXG7GnehNus-EW2FDuTzB8HKW_3HUjhPt9ZNf45w@mail.gmail.com> <87y3x7zh6h.fsf@vitty.brq.redhat.com>
 <alpine.DEB.2.20.1702161832020.3543@nanos> <87tw7txgx9.fsf@vitty.brq.redhat.com> <CALCETrXB8CufQujLAg6bbq=DGAMUE293CF7L4Kp+mCSoNWyuBg@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 Fri, 17 Feb 2017, Andy Lutomirski wrote:
> On Fri, Feb 17, 2017 at 2:14 AM, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> > Not only. As far as I understand (and I *think* K. Y. pointed this out)
> > when VM is migrating to another host TSC page clocksource is disabled for
> > extended period of time so we're better off reading from MSR than
> > looping here. With regards to VDSO this means reverting to doing normal
> > syscall.
> 
> That's a crappy design.  The guest really ought to be able to
> distinguish "busy, try again" from "bail and use MSR".
> 
> Thomas, I can see valid reasons why the hypervisor might want to
> temporarily disable shared page-based timing, but I think it's silly
> that it's conflated with indicating "retry".

It's beyond silly if that's the case.

> But if this is indeed the ABI, we're stuck with it.

No argument.