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

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Message-Id: <200712192225.53748.amit.shah@qumranet.com>
Date:	Wed, 19 Dec 2007 22:25:53 +0530
From:	Amit Shah <amit.shah@...ranet.com>
To:	kvm-devel@...ts.sourceforge.net
Cc:	"Glauber de Oliveira Costa" <glommer@...il.com>,
	"Avi Kivity" <avi@...o.co.il>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Avi Kivity <avi@...ranet.com>,
	Gerd Hoffmann <kraxel@...hat.com>
Subject: Re: [kvm-devel] Guest kernel hangs in smp kvm for older kernels prior to tsc sync cleanup

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?

> 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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