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

[Thomas Gleixner]

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