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

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Message-ID: <4C3E2806.1050900@goop.org>
Date:	Wed, 14 Jul 2010 14:11:34 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 01:50 PM, H. Peter Anvin wrote:
> 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.
>   

Yes.  I'd say write_crX should need that too, since they can they can
have a variety global effects on how memory addressing works (from tlb
flush to switching pagetable formats).

> 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.
>   

Hm.  Well we do *definitely* rely on that.  I guess technically we'd
need at least a memory input on the asm to be ordered with respect to a
memory clobber.  But that doesn't seem to have been an issue (or perhaps
its mixed up with the secret semantics of asm volatile).

    J
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