Re: [Xen-devel] [PATCH v8 15/17] x86/ctxt: Issue a speculation barrier between vcpu contexts

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Mon, 2018-01-15 at 14:23 +0100, David Woodhouse wrote:
> 
> > > 
> > > Also... if you're doing that in context_switch() does it do the right
> > > thing with idle? If a CPU switches to the idle domain and then back
> > > again to the same vCPU, does that do the IBPB twice?
> >
> > Context switches to idle will skip the IBPB because it isn't needed, but
> > any switch to non-idle need it.  In your example, we should execute just
> > a single IBPB.
> 
> In my example I think we should not execute IBPB at all. We come from a
> given VMCS, sleep for a while, and go back to it. No need for any
> flushing whatsoever.

msw points out that in my example we *don't* execute IBPB at all, I
think.

In both switching to idle, and back to the vCPU, we should hit this
case and not the 'else' case that does the IBPB:

1710     if ( (per_cpu(curr_vcpu, cpu) == next) ||
1711          (is_idle_domain(nextd) && cpu_online(cpu)) )
1712     {
1713         local_irq_enable();
1714     }

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