linux-kernel - Re: [PATCH 3/16] read/write_crX, clts and wbinvd for 64-bit paravirt

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Message-ID: <4729FB43.1010904@redhat.com>
Date:	Thu, 01 Nov 2007 14:13:55 -0200
From:	Glauber de Oliveira Costa <gcosta@...hat.com>
To:	Keir Fraser <Keir.Fraser@...cam.ac.uk>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>, lguest@...abs.org,
	kvm-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	ak@...e.de, chrisw@...s-sol.org, tglx@...utronix.de,
	anthony@...emonkey.ws, akpm@...ux-foundation.org,
	virtualization@...ts.linux-foundation.org, mingo@...e.hu
Subject: Re: [PATCH 3/16] read/write_crX, clts and wbinvd for 64-bit paravirt

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Keir Fraser escreveu:
> On 1/11/07 15:30, "Jeremy Fitzhardinge" <jeremy@...p.org> wrote:
> 
>> Glauber de Oliveira Costa wrote:
>>> I in fact have seen bugs with mixed reads and writes to the same cr,
>>> (cr4), but adding the volatile
>>> flag to the read function seemed to fix it.
>> Well, volatile will make a read be repeated rather than caching the
>> previous value, but it has no effect on ordering.
> 
> volatile prevents the asm from being 'moved significantly', according to the
> gcc manual. I take that to mean that reordering is not allowed.
> 
According to a gcc developer to whom I asked this question, volatile
prevents the code
to be removed, but does not prevent it to be moved (pun indented). In
practice, it should force
a re-read, but not influence the ordering decisions from the compiler.
Besides , 'significantly'
sounds like a significantly unprecise word, whose specific meaning may
be implementation dependant.

So I agree that adding a memory location reference is probably the best
alternative.

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