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

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Message-ID: <4C3E2304.5040805@zytor.com>
Date:	Wed, 14 Jul 2010 13:50:12 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Avi Kivity <avi@...hat.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 01:40 PM, Jeremy Fitzhardinge wrote:
> 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

invlpg, in the general case, definitely needs a memory clobber even if
volatiles are ordered, since it needs to be ordered with regards to
non-volatile memory operations.

Note that memory clobbers don't by themselves enforce ordering since
they don't prevent the ordering of memory clobbers with respect to each
other.

	-hpa

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