linux-kernel - Re: [RFC PATCH v3] Fix: clocksource watchdog marks TSC unstable on guest VM

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Date:	Wed, 9 Sep 2015 11:51:43 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Shaohua Li <shli@...com>
cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	John Stultz <john.stultz@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v3] Fix: clocksource watchdog marks TSC unstable on
 guest VM

On Tue, 8 Sep 2015, Shaohua Li wrote:
> On Tue, Sep 08, 2015 at 05:08:03PM +0200, Thomas Gleixner wrote:
> > For non paravirt kernels which can read the TSC directly, we'd need a
> > way to transport that information. A simple mechanism would be to
> > query an emulated MSR from the watchdog which tells the guest the
> > state of affairs on the host side. That would be a sensible and
> > minimal invasive change on both host and guests.
> 
> This will require every hypervisor supports the MSR, so not a solution
> we can expect immediately.

I know.
 
> I'm wondering why we can't just make the watchdog better to detect this
> watchdog wrap.

Again, I'm not opposed to make it better. I'm just trying to prevent
making the watchdog a total mess for no reason.

> It can happen in physical machine as I said before, but I
> can't find a simple way to trigger it, so it's not very convincing. But
> the watchdog doesn't work for specific environment (for exmaple, a bogus
> hardware doesn't responsond for some time) for sure, we shouldn't assume
> the world is perfect.

Sigh. If the damned hardware blocks long enough to wreckage the
watchdog then we have more serious problems than that.

Can you please stop this handwaving and provide some proper proof for
your arguments? I'm really tired of this.

Thanks,

	tglx
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