Re: [Xen-devel] Linux questions

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> 03.12.07 19:32 >>>
>On 3/12/07 11:40, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:
>
>>> rmb() is more powerful than barrier(), not the converse.
>> 
>> Oh, sorry, I mixed barrier() with mb(). So the proposal would then simply
>> be the other way around (the use of locked operations or fence instructions
>> on x86 is really unnecessary as long as WC memory or non-temporal stores
>> don't need to be taken into consideration).
>
>Then the implementation of rmb() should be equivalent to barrier(). The code
>in time-xen.c is implemented to the interface definitions of barrier() and
>rmb() -- the former is used just where instruction ordering is important;
>the latter where dynamic execution order matters too.

I have to disagree: At least the uses of barrier() in monotonic_clock() appear
to be in places where in reality (and from a theoretical standpoint) rmb()
ought to be used.

But I agree that rmb() (and also wmb()) on x86 doesn't need to be more
than barrier() (except, as said, in the context of WC memory or non-temporal 
memory accesses) - isn't that exactly what you just recently did in the
hypervisor?

Jan



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