On Tue, 2018-02-13 at 10:21 +0000, David Woodhouse wrote:
> 
> > So the right logic is:
> > 
> > - if the VM has IBRS_ALL, pass through the MSR when it is zero and
> > intercept writes when it is one (no writes should happen)
> > 
> > - if the VM doesn't have IBRS_ALL, do as we are doing now, independent
> > of what the host spectre_v2_ibrs_all() setting is.
> 
> We end up having to turn IBRS on again on vmexit then, taking care that
> no conditional branch can go round it. So that becomes an
> *unconditional* wrmsr or lfence in the vmexit path. We really don't
> want that.

Note that being able to keep it simple in KVM was basically what made
the difference between me tolerating IBRS_ALL as Intel currently define
it, and throwing my toys out of the pram (as I had done in the first
iterations of this patch).

If we *can't* keep it nice and simple in KVM, then I think we *really*
want Intel to make the hardware bit a no-op. If not for all CPUs with
the IBRS_ALL bit, then for all *future* CPUs and they can define a new
IBRS_ALL_UNCONDITIONAL bit. Which is the only one we'd ever care about.