linux-kernel - Re: [PATCH 1/2] x86: AMD: mark TSC unstable on APU family 15h models 10h-1fh

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Message-ID: <20140801090228.GA3924@nazgul.tnic>
Date:	Fri, 1 Aug 2014 11:02:28 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org, hpa@...or.com,
	mingo@...hat.com, tglx@...utronix.de, pbonzini@...hat.com,
	kvm@...r.kernel.org, mtosatti@...hat.com
Subject: Re: [PATCH 1/2] x86: AMD: mark TSC unstable on APU family 15h models
 10h-1fh

On Thu, Jul 31, 2014 at 09:47:12AM +0000, Igor Mammedov wrote:
> Due to erratum #778 from
> "Revision Guide for AMD Family 15h Models 10h-1Fh Processors,
>  Publication # 48931, Issue Date: May 2013, Revision: 3.10"
> 
> TSC on affected processor, a core may drift under certain conditions,
> which makes initially synchronized TSCs to become unsynchronized.
> 
> As result TSC clocksource becomes unsuitable for using as wallclock
> and it brakes pvclock when it's running with PVCLOCK_TSC_STABLE_BIT
> flag set.
> That causes backwards clock jumps when pvclock is first read on
> CPU with drifted TSC and then on CPU where TSC was stable or had
> a lower drift rate.
> 
> To fix issue mark TSC as unstable on affected CPU, so it won't
> be used as clocksource. Which in turn disables master_clock
> mechanism in KVM and force pvclock using global clock counter
> that can't go backwards.
> 
> Signed-off-by: Igor Mammedov <imammedo@...hat.com>

Acked-by: Borislav Petkov <bp@...e.de>
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