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

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Date:	Fri, 27 Apr 2007 00:08:07 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <ak@...e.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	virtualization@...ts.osdl.org, lkml <linux-kernel@...r.kernel.org>,
	Ian Pratt <ian.pratt@...source.com>,
	Christian Limpach <Christian.Limpach@...cam.ac.uk>,
	Adrian Bunk <bunk@...sta.de>
Subject: Re: [PATCH 06/25] xen: Core Xen implementation

Andi Kleen wrote:
>> +	/* convert from IF type flag */
>> +	flags = !(flags & X86_EFLAGS_IF);
>> +	vcpu = x86_read_percpu(xen_vcpu);
>> +	vcpu->evtchn_upcall_mask = flags;
>> +	if (flags == 0) {
>> +		barrier(); /* unmask then check (avoid races) */
>>     
>
> 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.

    J
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