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

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> > Can someone at Intel confirm or deny that VMware ESX
> > always traps rdtsc?  If so, it is probably not hard
> > to write an application that works on VMware ESX (on
> > certain hardware) but fails on Xen.
> 
> I'd be rather surprised if VMware trapped RDTSC. From what I 
> gather, ESX3 doesn't make a great deal of use of VT for 32b 
> guests, so at the very least it would be tricky to do 
> anything about user space use of rdtsc.

Some googling and reading provides evidence that VMware
does indeed virtualize the TSC.  The timekeeping paper
http://www.vmware.com/pdf/vmware_timekeeping.pdf
tells how to turn vTSC off, but says that turning it
off is no longer recommended.  The ASPLOS paper
http://www.vmware.com/pdf/asplos235_adams.pdf
uses rdtsc as an example of how binary translation
is much faster than emulation or callout (though
their BT version fetches a stale TSC which afaict
doesn't solve the ordering problem).

Also, Avi Kivity tells me that the KVM folks have
also recently come to the conclusion that it is necessary
to emulate TSC, though KVM currently does not.

Dan

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