RE: [Xen-devel] [PATCH] new nomigrate config option

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

Oops, sorry I forgot to add perhaps the most important
part of the idea below:

If the physical system has the Invariant TSC cpuid
bit set AND a domain has nomigrate==1, tsc_native
could be auto-enabled for that domain!

> -----Original Message-----
> From: Dan Magenheimer 
> Sent: Thursday, October 22, 2009 11:57 AM
> To: Keir Fraser; Tim Deegan
> Cc: Xen-Devel (E-mail)
> Subject: RE: [Xen-devel] [PATCH] new nomigrate config option
> 
> 
> > >> I suspect Dan wants to interrogate this flag from 
> unprivileged domU
> > >> userspace, with no domU kernel modifications.
> > > 
> > > In that case all that's needed is a user-space xenbus client. :)
> > 
> > Is that all. :-)
> > 
> > I plan to see how this userspace hypercall stuff goes and at 
> > the end of the
> > day, if/when we don't like it, it's going to be very easy 
> to disable.
> 
> Actually, I've got an interesting idea that requires no
> userspace hypercall.
> 
> If the physical system has the Invariant TSC cpuid bit set
> AND a domain has nomigrate==1, the Invariant TSC bit can/should
> be surfaced to the domain.  Also, Invariant TSC can/should
> be surfaced to ANY domain that is booted with tsc_native==0.
> 
> Then apps that test for the Invariant TSC bit (and find it
> set) can always use rdtsc directly.
> 
> More complicated environments (e.g. where high-frequency time
> stamp apps are running and migration is still required) may
> still need userspace hypercall, but this first step can
> still be useful on many physical systems without them.
> 
> Dan
> 
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