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

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Date:   Fri, 10 Feb 2017 19:01:52 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     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>,
        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

On Fri, 10 Feb 2017, Stephen Hemminger wrote:

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

That's a different question and can only be answered by the hypervisor
folks. Dunno, whether they have barrier requirements. The seqcount stuff
relies on:

do {
	seq = READ_ONCE(s->sequence);

	smp_rmb();

	sample_data();
  
	smp_rmb();
} while (s->sequence != seq);

which pairs with the writer side:

       s->sequence++;
       
       smp_wmb();

       update_data();

       smp_wmb();

       s->sequence++;

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.

Thanks,

	tglx