[Xen-devel] RE: TSC scaling and softtsc reprise, and PROPOSAL

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> >> I believe this affects the guest OS executing RDTSC, not
> >> guest apps, and is
> >> only to delay the TSC to not 'run past' pending timer ticks
> >> (typically where
> >> they have been delayed due to the guest being preempted).
> > 
> > Could be.  The text would lead me to believe otherwise
> > though.  Read the section on "Virtual TSC" in the
> > above PDF. Specifically the Virtual TSC "advances even
> > when the the virtual CPU is not running" and "In the
> > past, this feature had sometimes been recommended to
> > improve performance of APPLICATIONS that read the
> > TSC frequently..." (my emphasis)
> 
> Yes, then it sounds like they virtualise it for apps too. 
> Also there is an
> option to virtualise the TSC at a specified frequency -- that would be
> pretty weird if it applied only to guest-OS RDTSCs but not 
> guest-app RDTSCs.
> 
> Interesting...
> 
>  -- Keir

And further, the frequency is "sticky" across migration, with
the frequency set to whatever machine the VM originated on.

I'd be inclined to just use Xen system time and thus
the TSC frequency would be always 1GHz on all systems.

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