RE: [Xen-devel] RE: rdtsc: correctness vs performance on Xen (and KVM?)

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> 01.09.09 17:56 >>>
>Can you think of any trick (that doesn't require the cost of a
>trap/hypercall) to allow an app to determine what pcpu
>it is running on?

Just like what is being used to allow apps to get the CPU number on native
kernels (or the vCPU one on Xen-ified ones): Have a GDT entry the limit of
which is the number you want, and have the app use the lsl instruction to
get at it.

I am, however, always a little bit concerned when it comes to exposing
information that shouldn't really be exposed, due to the possibility of
overlooking potential misuses. In the specific case here, I can't see at all
why you'd the pCPU number exposed - after all the kernel can do what
you want apps to do without having that information.

Jan


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