Attempts to set up a 64-bit guest operating system using VMware workstation on a 64-bit Intel processor machine running Windows 2003 R2 Standard 64-bit Edition failed miserably this past week at work. As Chaz6 and larstr subsequently pointed out in #vmware on irc.freenode.net, the particular Intel processor in the machine being used does not support Virtulisation Technology (VT). It came as a thudding surprise as admittedly I had not known anything about VT and suspected that VMware would not ever need rely on a special [hardware] feature of 64-bit processors to be able to emulate 64-bit guest operating systems, but as things stand today, VMware cannot. I had discussion soon thereafter with Talha over the slightly shocking yet disappointing discovery, which mostly fell into a heated debate as all serious discussions with Talha do — he apparently mostly can’t seem to be able to take rigorous positive criticism in stride. However, he pointed out a link to a knowledge base article from VMware that explains sufficiently clearly why VMware has to confront this particular requirement.
I have my reservations about Xen being able to overcome this restriction in order to emulate 64-bit operating systems on 64-bit hardware without VT support (or the AMD equivalent). However, a rather pissed-off soul in ##xen on irc.freenode.net did, when I inquired after the issue, briefly comment that Xen should not have any problems. Beyond that, further inquisitions did not yield so much as a response from anyone. I may probably end up putting 64-bit version of some Linux on that machine and pulling off setting up Xen on it.
Additionally, if you are playing with VMware on 64-bit hardware, there are tools that will check beforehand for you whether the processor supports VT (or AMD equivalent) or not.