From mboxrd@z Thu Jan  1 00:00:00 1970
From: Thomas Gleixner <tglx@linutronix.de>
Subject: RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read
 method
Date: Mon, 13 Feb 2017 10:27:18 +0100 (CET)
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To: Dexuan Cui <decui@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>, Haiyang Zhang <haiyangz@microsoft.com>, "x86@kernel.org" <x86@kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Andy Lutomirski <luto@amacapital.net>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>, "virtualization@lists.linux-foundation.org" <virtualization@lists.linux-foundation.org>
List-Id: virtualization@lists.linuxfoundation.org

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