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

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Message-ID: <4C3DF1BE.2070404@goop.org>
Date:	Wed, 14 Jul 2010 10:19:58 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Palfrader <peter@...frader.org>,
	Avi Kivity <avi@...hat.com>, 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 07/13/2010 05:15 PM, Linus Torvalds wrote:
> The gcc documentation wrt inline asm's is totally worthless. Don't
> even bother quoting it - because the gcc people themselves have never
> cared. If the docs ever end up not matching what they want to do, they
> will just change the documentation.
>
> In other words, at least historically the docs are not in any way
> meaningful. They are not a "these are the semantics we guarantee",
> they are just random noise. As I mentioned, the docs historically just
> said something like "will not be moved significantly", and apparently
> they've been changed to be something else.
>
>   

Sure, I completely agree.  At the moment the docs say "asm volatile
guarantees nothing", and we can work with that.  So long as we don't
expect asm volatile to mean anything more (ie, magic semantics involving
reordering), everyone is happy.

BTW, gcc 2.95's docs do mention "asm volatile" having an effect on
ordering, which is probably where the notion came from: "If you write an
`asm' instruction with no outputs, GNU CC [...] not delete the
instruction or move it outside of loops. [...] you should write the
`volatile' keyword to prevent future versions of GNU CC from moving the
instruction around within a core region".  Lucky we never relied on
that, right?  Right?

    J
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