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

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> >> 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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