[Xen-devel] Re: rdtsc hypercall, from userland?!? (was: rdtsc: correctness vs performance on Xen)

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> 09.09.09 20:05 >>>
>(Note these measurements are normal kernel-land
>hypercalls.)  Currently all hypercalls from userland
>are illegal, but this need not be the case for ALL
>hypercalls. Is it possible
>for Xen to implement a "rdtsc hypercall" that
>is executable from userland, without requiring
>OS changes?  Early discussions look promising.

While possible, I'd suspect that the good performance you see for
64-bits wouldn't hold: You can't (without potential for ambiguity)
re-use syscall for this purpose, and the alternative ways (interrupt
or call gate) are likely to be more in the performance range of what
you measured for 32-bits, which doesn't seem that much better
than emulated rdtsc.

Jan


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