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

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Date:   Mon, 13 Feb 2017 20:28:21 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>
cc:     Dexuan Cui <decui@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>, 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, Andy Lutomirski wrote:

> On Sun, Feb 12, 2017 at 11:49 PM, Dexuan Cui <decui@...rosoft.com> 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.
> >>
> >>       tglx
> >
> > 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.
> 
> If the patch is correct, there is one of these shared by all vCPUs, so
> this is not a sufficient explanation.

Right, because that's ony ONE TSC page for the whole guest and then the seq
stuff really lacks barriers.

Thanks,

	tglx