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Date: Mon, 13 Feb 2017 10:27:18 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Dexuan Cui <decui@microsoft.com>
cc: Stephen Hemminger <sthemmin@microsoft.com>,
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Subject: RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read
 method
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On Mon, 13 Feb 2017, Dexuan Cui wrote:
> > From: Thomas Gleixner [mailto:tglx@linutronix.de]
> > Sent: Saturday, February 11, 2017 02:02
> >  ...
> > That's important if the stuff happens cross CPU. If the update happens on
> > the same CPU then this is a different story and as there are VMexits
> > involved they might provide the required ordering already. But I can't tell
> > as I have no idea how that host side thing is done.
> > 
> 
> IMO Hyper-V TSC page clocksource here seems pretty similar to KVM's pvclock,
> So I would guess "the structure is only updated just before reentering the guest
> after some VM event" (https://rwmj.wordpress.com/2010/10/15/kvm-pvclock/),
> that is, the update should happen on the same CPU, I guess.

Guessing does not help much. There are a gazillion ways to implement such a
thing. I'm sure that there exists proper documentation of the mechanism
inside your company.

Thanks,

	tglx