Re: x86-64 tools fix question

[Anthony Liguori <aliguori@us.ibm.com>]

David Hopwood wrote:

> Jerone Young wrote:
> See volume 1, section 3.9.2 of the AMD64 architecture manual
> (http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252_875_7044,00.html), 
>
> quoted below for convenience.
>
> This should be guaranteed to work:
>
> #define rmb() __asm__ __volatile__ ( "mfence" : : : "memory" )
> #define wmb() __asm__ __volatile__ ( "mfence" : : : "memory" )

sfence and lfence would seem to be more appropriate.  mfence would seem 
to be appropriate for an mb() macro (which is usually also defined--but 
in this case not necessary).

> but may be overkill. Are these macros used in any performance-critical
> situations? If not then the overkill wouldn't matter.

Not at all.  They're currently used to prevent race conditions within 
the domain control channel ring queue.  This queue is relatively 
low-traffic.

The confusion stemmed from the x86-32 code that doesn't actually 
implement a wmb() since no relevant architecture does out-of-order stores.

x86-64 is another story.  The linux includes still have the same 
comments indicating that wmb() is not necessary (an obvious 
cut-and-paste job) even though wmb() is defined to sfence.  To make 
matters worse, the wmb() is wrapped in the same CONFIG_UNORDERED_IO 
guarded (that's enabled by default in x86-64 but disabled by default in 
x86-32).

It's understandably confusing.  I'm hoping we can do a much better job 
in Xen in supporting both architectures to avoid these sort of confusions.

Regards,
Anthony Liguori


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