On Mon, 2018-01-29 at 12:44 -0800, Arjan van de Ven wrote:
> On 1/29/2018 12:42 PM, Eduardo Habkost wrote:
> > 
> > The question is how the hypervisor could tell that to the guest.
> > If Intel doesn't give us a CPUID bit that can be used to tell
> > that retpolines are enough, maybe we should use a hypervisor
> > CPUID bit for that?
>
> the objective is to have retpoline be safe everywhere and never use IBRS
> (Linus was also pretty clear about that) so I'm confused by your question

The question is about all the additional RSB-frobbing and call depth
counting and other bits that don't really even exist for Skylake yet in
a coherent form.

If a guest doesn't have those, because it's running some future kernel
where they *are* implemented but not enabled because at *boot* time it
discovered it wasn't on Skylake, the question is what happens if that
guest is subsequently migrated to a Skylake-class machine.

To which the answer is obviously "oops, sucks to be you". So yes,
*maybe* we want a way to advertise "you might be migrated to Skylake"
if you're booted on a pre-SKL box in a migration pool where such is
possible. 

That question is a reasonable one, and the answer possibly the same,
regardless of whether the plan for Skylake is to use IBRS, or all the
hypothetical other extra stuff.