linux-kernel - RE: [PATCH 2/2] x86/vdso: Add VCLOCK

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Date:   Mon, 13 Feb 2017 10:27:18 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dexuan Cui <decui@...rosoft.com>
cc:     Stephen Hemminger <sthemmin@...rosoft.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>
Subject: RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read
 method

On Mon, 13 Feb 2017, Dexuan Cui wrote:
> > From: Thomas Gleixner [mailto:tglx@...utronix.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