On Thu, 22 Nov 2018, Thomas Gleixner wrote:
> On Thu, 22 Nov 2018, Borislav Petkov wrote:
> > > +
> > > + /* Initialize Indirect Branch Prediction Barrier */
> > > + if (boot_cpu_has(X86_FEATURE_IBPB)) {
> > > + setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
> > > + pr_info("Spectre v2 mitigation: Enabling Indirect Branch
> > > Prediction Barrier\n");
> > > + }
> >
> > So AFAICT, if coming in here with AUTO, we won't enable IBPB and I
> > *think* AMD wants IBPB enabled. At least the whitepaper says:
> >
> > "IBPB combined with Reptoline software support is the AMD recommended
> > setting for Linux mitigation of Google Project Zero Variant 2
> > (Spectre)."
>
> Ok. That's indeed a step backwards, because we don't do IBPB in KVM
> anymore. I'll fix that tomorrow morning when brain is more awake.
OTOH, off means that all of it is disabled. Which was the case already
before this when spectre_v2=off is on the command line.
Now with the default to prctl/seccomp the IBPB in KVM is enabled. So no
change there.
> IBPB on context switch is controlled separately anyway now, so that's a
> nobrainer to sort out.
>
> Though I wait for Toms answer whether we really want IBPB on context switch
> for AMD by default.
That still stands. But if we want to do that, then we need to optimize it a
bit. Isn't that hard, but ...
Thanks,
tglx
