linux-kernel - Re: [PATCH 06/25] xen: Core Xen implementation

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Date:	Fri, 27 Apr 2007 08:31:23 +0100
From:	Keir Fraser <Keir.Fraser@...cam.ac.uk>
To:	Jeremy Fitzhardinge <jeremy@...p.org>, Andi Kleen <ak@...e.de>
CC:	Ian Pratt <ian.pratt@...source.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Adrian Bunk <bunk@...sta.de>, <virtualization@...ts.osdl.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 06/25] xen: Core Xen implementation

On 27/4/07 08:08, "Jeremy Fitzhardinge" <jeremy@...p.org> wrote:

>> Don't you need a rmb() here then? The CPU could speculate reads
>> (more occurrences)
>>   
> 
> Is rmb() sufficient?  It will stop a speculative read on the pending
> flag, but will it make sure the write has happened by then?  Ie, is it a
> write-vs-read barrier, or just a read-vs-read?
> Documentation/memory-barriers.txt suggests not.

The barrier() is sufficient. We are racing against Xen checking
evtchn_upcall_mask *on the local cpu*. Which means an interrupt has to
occur, which squashes speculative stuff.

Yeah, I know, it needs a better comment. :-)

 -- Keir


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