Re: [patch 15/24] x86/speculation: Add command line control for indirect branch speculation

[Thomas Gleixner]

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.

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.

Thanks,

	tglx