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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

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
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.

 -- Keir



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel