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

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Date:	Wed, 14 Jul 2010 10:45:05 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Avi Kivity <avi@...hat.com>, "H. Peter Anvin" <hpa@...or.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>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [patch 134/149] x86, paravirt: Add a global synchronization point
 	for pvclock

On 07/14/2010 10:40 AM, 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
>    

clts() has no memory clobber; it is used to serialize execution of code 
within kernel_fpu_begin() / kernel_fpu_end() blocks.

If the code within is reordered before the clts(), we've corrupted guest 
FPU state.

That's the kind of bug I think Linus is talking about.  We've been 
expecting volatile to work that way for over a decade, by my 
recollection, and if it doesn't, there is going to be a lot of broken code.

Shouldn't we at least get a compiler switch to force the volatile 
behavior?  I'd suggest it default to conservative.
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