linux-kernel - Re: [patch 134/149] x86, paravirt: Add a global synchronization point for pvclock

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Date:	Wed, 14 Jul 2010 14:11:23 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Palfrader <peter@...frader.org>,
	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	stable@...nel.org, stable-review@...nel.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Glauber Costa <glommer@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [patch 134/149] x86, paravirt: Add a global synchronization point
 	for pvclock

On 07/14/2010 01:45 PM, Zachary Amsden wrote:
> clts() has no memory clobber; it is used to serialize execution of
> code within kernel_fpu_begin() / kernel_fpu_end() blocks.
>
> If the code within is reordered before the clts(), we've corrupted
> guest FPU state.

Hm, that's awkward - you'd really need some way of specifying ordering
with respect to the FPU state, and "memory" would only be an approximate
proxy of that.  And not a good one, since gcc wouldn't regard
register-only fpu ops as being affected by a memory clobber.

    J
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