Ingo Molnar wrote:

ugh. Please take it from me: i've watched the Linux time code walk its 
long, rocky 10+ years road. One of the first mistakes was when we made 
the TSC the center of the i386-time universe. (incidentally, it was me 
who did the first steps of that, as a rookie kernel hacker) We got cured 
out of that in v2.6.19, v2.6.20 and v2.6.21. Granted, Xen is only at the 
beginning of that same road. Meet in another 10 years? ;)

Yep, the tsc has myriad problems; for Xen its the best of a bad lot. 
Unfortunately in 10 years no clearly better alternative has appeared;
maybe in 10 years there will be one.  It might even be the tsc.

    J