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

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Message-ID: <4C3CFB8B.1090804@goop.org>
Date:	Tue, 13 Jul 2010 16:49:31 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 03:14 PM, H. Peter Anvin wrote:
> Actually, I believe volatile operations (including asm volatile) are
> strictly ordered *with respect to other volatile operations*. 

The documentation makes no reference to that property; in fact it
suggests it is outright not true:

    Note that even a volatile `asm' instruction can be moved relative to
    other
    code, including across jump instructions.  For example, on many targets
    there is a system register which can be set to control the rounding
    mode of floating point operations.  You might try setting it with a
    volatile `asm', like this PowerPC example:

                asm volatile("mtfsf 255,%0" : : "f" (fpenv));
                sum = x + y;

    This will not work reliably, as the compiler may move the addition
    back before the volatile `asm'.  To make it work you need to add an
    artificial dependency to the `asm' referencing a variable in the
    code you don't want moved, for example:

             asm volatile ("mtfsf 255,%1" : "=X"(sum): "f"(fpenv));
             sum = x + y;

    Similarly, you can't expect a sequence of volatile `asm'
    instructions to remain perfectly consecutive.
    [...]
    An `asm' instruction without any output operands will be treated
    identically to a volatile `asm' instruction.

>  As such I
> would think we'd want to keep the "memory" clobber here, to make it
> strictly ordered with regards to *all* memory operations.
>   

That would keep its overall effect consistent.

    J
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