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

Zachary Amsden
2010-07-14T20:46:01Z

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.