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xen-devel
Re: [Xen-devel] x86-64 tools fix question
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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