linux-kernel - Re: [kvm-devel] Guest kernel hangs in smp kvm for older kernelsprior to tsc sync cleanup

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Date:	Wed, 19 Dec 2007 19:04:53 +0200
From:	Dor Laor <dor.laor@...il.com>
To:	Amit Shah <amit.shah@...ranet.com>
CC:	kvm-devel@...ts.sourceforge.net, Avi Kivity <avi@...ranet.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Avi Kivity <avi@...o.co.il>, Gerd Hoffmann <kraxel@...hat.com>
Subject: Re: [kvm-devel] Guest kernel hangs in smp kvm for older kernelsprior
 to tsc sync cleanup

Amit Shah wrote:
>
> On Wednesday 19 December 2007 21:02:06 Glauber de Oliveira Costa wrote:
> > On Dec 19, 2007 12:27 PM, Avi Kivity <avi@...o.co.il> wrote:
> > > Ingo Molnar wrote:
> > > > * Avi Kivity <avi@...ranet.com> wrote:
> > > >> Avi Kivity wrote:
> > > >>>  Testing shows wrmsr and rdtsc function normally.
> > > >>>
> > > >>> I'll try pinning the vcpus to cpus and see if that helps.
> > > >>
> > > >> It does.
> > > >
> > > > do we let the guest read the physical CPU's TSC? That would be 
> trouble.
> > >
> > > vmx (and svm) allow us to add an offset to the physical tsc.  We 
> set it
> > > on startup to -tsc (so that an rdtsc on boot would return 0), and
> > > massage it on vcpu migration so that guest rdtsc is monotonic.
> > >
> > > The net effect is that tsc on a vcpu can experience large forward 
> jumps
> > > and changes in rate, but no negative jumps.
> >
> > Changes in rate does not sound good. It's possibly what's screwing up
> > my paravirt clock implementation in smp.
>
> Do you mean in the case of VM migration, or just starting them on a single
> host?
>
It's the cpu preemption stuff on local host and not VM migration
>
> > Since the host updates guest time prior to putting vcpu to run, two
> > vcpus that start running at different times will have different system
> > values.
> >
> > Now if the vcpu that started running later probes the time first,
> > we'll se the time going backwards. A constant tsc rate is the only way
> > around
> > my limited mind sees around the problem (besides, obviously, _not_
> > making the system time per-vcpu).
>
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