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

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Date:	Tue, 13 Jul 2010 17:02:42 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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

I believe that's how it is actually implemented though, and I believe we'd have failures all over if that was not true.

"Jeremy Fitzhardinge" <jeremy@...p.org> wrote:

>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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