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

H. Peter Anvin
2010-07-14T20:51:10Z

On 07/14/2010 01:40 PM, 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

invlpg, in the general case, definitely needs a memory clobber even if
volatiles are ordered, since it needs to be ordered with regards to
non-volatile memory operations.

Note that memory clobbers don't by themselves enforce ordering since
they don't prevent the ordering of memory clobbers with respect to each
other.

-hpa