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

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Message-ID: <4C3E20B3.6020007@goop.org>
Date:	Wed, 14 Jul 2010 13:40:19 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
CC:	"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>,
	Zachary Amsden <zamsden@...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 01:16 PM, Avi Kivity wrote:
> On 07/14/2010 08:57 PM, Jeremy Fitzhardinge wrote:
>> Anything else?
>
> 1. set up a mapping
> 2. invlpg or set cr3
> 3. use the mapping
>
> Moving the invlpg will break your code.

invlpg uses memory clobbers.  All the crX ops seem to use a
__force_order variable to sequence them - but it looks like it's done
precisely backwards and it's barking mad to do allow write_crX to be
reordered with respect to memory ops.

Hm, looks like glommer added it surreptitiously while unifying
system_32.h and system_64.h (system_32.h relied on asm volatile not
being reordered; system_64.h used memory clobbers).
    J
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