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

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Message-ID: <BLUPR0301MB2098E1283D42BD2C3772D839CC440@BLUPR0301MB2098.namprd03.prod.outlook.com>
Date:   Fri, 10 Feb 2017 16:31:21 +0000
From:   Stephen Hemminger <sthemmin@...rosoft.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
CC:     "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>,
        Dexuan Cui <decui@...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

Since sequence count algorithm is done by hypervisor, better to not reuse seqcount.
Still concerned that the code is racy.

-----Original Message-----
From: Thomas Gleixner [mailto:tglx@...utronix.de] 
Sent: Friday, February 10, 2017 4:28 AM
To: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: Stephen Hemminger <sthemmin@...rosoft.com>; 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>; Dexuan Cui <decui@...rosoft.com>; linux-kernel@...r.kernel.org; devel@...uxdriverproject.org; virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

On Fri, 10 Feb 2017, Vitaly Kuznetsov wrote:

> Stephen Hemminger <sthemmin@...rosoft.com> writes:
> 
> > Why not use existing seqlock's?
> >
> 
> To be honest I don't quite understand how we could use it -- the 
> sequence locking here is done against the page updated by the 
> hypersior, we're not creating new structures (so I don't understand 
> how we could use struct seqcount which we don't have) but I may be 
> misunderstanding something.

You can't use seqlock, but you might be able to use seqcount. Though I doubt it given the 0 check ....

Thanks,

	tglx