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

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Message-ID: <MWHPR03MB266952220A7E1FA0F2D38453BF590@MWHPR03MB2669.namprd03.prod.outlook.com>
Date:   Mon, 13 Feb 2017 07:49:26 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Stephen Hemminger <sthemmin@...rosoft.com>
CC:     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

> 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.

Thanks,
-- Dexuan