RE: [Xen-devel] rdtscP and xen (and maybe the app-tsc answer I've been looking for)

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> 22.09.09 01:29 >>>
>Yes, I neglected an important pre-condition.  ASSUME the first
>rdtscp on pcpu-A gets a version mismatch so that it must fetch
>the parameters again.  Then: the vcpu switches pcpu TWICE
>from pcpu-A to pcpu-B and back to pcpu-A and does rdtscp
>each time on pcpu-A but reads one or more pvclock parameters
>(that are too big to be encoded in TSC_AUX) on pcpu-B.

This fundamentally depends on how the pvclock parameters are being
read: While app-accessible MSRs inherently require each of the necessary
RDMSRs to be executed on the correct {p,v}CPU (unless you encode the
CPU number in the RDMSR input), an app accessible shared memory region
wouldn't have that property.

Jan


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