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RE: [Xen-devel] HPET in HVM guests

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-devel] HPET in HVM guests
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Fri, 18 Dec 2009 09:55:42 -0800 (PST)
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IIRC HPET required some timer_mode work because certain
kernel versions had problems when ticks were delivered
too closely together.  Dave Winchell did this at Virtual
Iron, but left early this year, and his work was
plumbed all the way through to the hardware HPET
(because it preceded work that made xen system time
more reliable) so resurrecting his VI patch and
munging it to apply to xen-unstable may take significant
work and testing (that I may have to eventually get 
around to, but not soon).

Are you suggesting that an HVM use of HPET should be
plumbed through to the hardware HPET?  Hardware HPET
access is verrrrryyyyyy slow and save/restore/migration
would likely be a problem.

And in any case, what do you expect to gain by using
HPET instead of another clocksource?

Turning on vhpet by default (on top of Xen system time)
may turn out to be OK, but unless there's a really good
reason and a great deal of testing done soon, I wouldn't
suggest flipping the default prior to 4.0.


> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Sent: Friday, December 18, 2009 9:38 AM
> To: Jan Beulich; xen-devel@xxxxxxxxxxxxxxxxxxx
> Cc: Dan Magenheimer
> Subject: Re: [Xen-devel] HPET in HVM guests
> Due to xen-unstable:17017. Now that HPET model is integrated 
> with vpt.c
> better, I think we could enable vhpet by default.
>  -- Keir
> On 18/12/2009 16:18, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
> > Is there a reason HPET is not by default turned on for HVM 
> guests? While
> > Xen itself sets it to on, the tools - afaics - default it 
> back to off.
> > 
> > Thanks, Jan
> > 
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel

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