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

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 01.09.09 23:25 >>>
>On 01/09/2009 17:04, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
>
>>>>> 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.
>
>Yes, that's true. Xen could provide such a segment descriptor in its private
>area of the GDT. The issue then would be that, in a compound pvclock

And in fact there already is such a descriptor, just with DPL=0.

>operation spanning multiple machine instructions, the pCPU number revealed
>by the LSL instruction can be stale by the time it is used later in the
>compound operation.

Correct.

Jan



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