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

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Jeremy Fitzhardinge <jeremy@xxxxxxxx> 09.09.09 20:59 >>>
>You could enable/disable emulation rdtsc each context switch according
>to the app's desires/requrements.  It would require an extra hypercall
>per context switch, but it could be batched with the others, resulting
>in little marginal cost.  It would, however, leave the kernel's use of
>rdtsc in a confused state.

Not necessarily: There could be two flags, one saying app rdtsc needs to
be emulated, and a second one for the kernel ones'. Unless a pv kernel
wants this, its (emulated) reads could still return the real (hardware)
value rather than the calculated, 1GHz-based one.

Jan


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