On Fri, 28 Feb 2014, Qin Li wrote:
> 于 2014/2/27 21:03, Stefano Stabellini 写道:
> > > >I guest linux guest will do the same thing, rdtsc() fetch current
> > > timestamp from current running vCPU, TSC out-of-sync issue is still there.
> > > >It seems to me pvclock finally fix the time drift issue just because the
> > > workaround enforced as above, right?
> > First you should know that TSC is not always guaranteed to be
> > synchronized across multiple processors, especially on older systems.
> > On "TSC-safe" systems, Xen would export a consistent TSC to guests, by
> > setting the vtsc offset and scale appropriately.
> Stefano,
> 
> Is there a easy way to check whether the system is "TSC-safe"? Do you mean
> "X86_FEATURE_TSC_RELIABLE" bit in "boot_cpu_data.x86_capability"?

Yes, I think that's it.
Maybe you want to read docs/misc/tscmode.txt.