linux-kernel - Re: [Xen-devel] [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush

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Message-ID: <20170815200232.3ad8139e@gandalf.local.home>
Date:   Tue, 15 Aug 2017 20:02:32 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        boris.ostrovsky@...cle.com,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>,
        Jork Loeser <Jork.Loeser@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Simon Xiao <sixiao@...rosoft.com>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        KY Srinivasan <kys@...rosoft.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "mingo@...nel.org" <mingo@...nel.org>
Subject: Re: [Xen-devel] [tip:x86/platform] x86/hyper-v: Use hypercall for
 remote TLB flush

On Fri, 11 Aug 2017 14:07:14 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> It goes like:
> 
> 	CPU0			CPU1
> 
> 	unhook page
> 				cli
> 				traverse page tables
> 	TLB invalidate --->	<IF clear, therefore CPU0 waits>
> 				sti
> 				<IPI>
> 				 TLB invalidate
> 			<------	 complete

I guess the important part here is the above "complete". CPU0 doesn't
proceed until its receives it. Thus it does act like
cli~rcu_read_lock(), sti~rcu_read_unlock(), and "TLB invalidate" is
equivalent to synchronize_rcu().

[ this response is for clarification for the casual observer of this
  thread ;-) ]

-- Steve

> 				</IPI>
> 	free page
> 
> So the CPU1 page-table walker gets an existence guarantee of the
> page-tables by clearing IF.