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

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Date:	Thu, 01 Nov 2007 08:30:59 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Glauber de Oliveira Costa <gcosta@...hat.com>
CC:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	rusty@...tcorp.com.au, ak@...e.de, mingo@...e.hu,
	chrisw@...s-sol.org, avi@...ranet.com, anthony@...emonkey.ws,
	virtualization@...ts.linux-foundation.org, lguest@...abs.org,
	kvm-devel@...ts.sourceforge.net, zach@...are.com,
	tglx@...utronix.de, jun.nakajima@...el.com, glommer@...il.com,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 3/16] read/write_crX, clts and wbinvd for 64-bit paravirt

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.

> Yet, I agree with you that
> the theorectical problem exists for the reorder, and your proposed fix
> seems fine (although if we're really desperate about memory usage, we
> can use a char instead a int and save 3 bytes!)

Sure.  Ideally the compiler would never even generate a reference to it,
and it could just be extern, but in practice the compiler will generate
references sometimes.

    J
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