Re: [PATCH 3/7] x86/bugs: Fix BHI handling of RRSBA

[Josh Poimboeuf]

On Thu, Apr 11, 2024 at 11:02:42AM +0100, Andrew Cooper wrote:
> >  /* Disable in-kernel use of non-RSB RET predictors */
> >  static void __init spec_ctrl_disable_kernel_rrsba(void)
> >  {
> > -	u64 ia32_cap;
> > +	if (rrsba_disabled)
> > +		return;
> > +
> > +	if (!(ia32_cap & ARCH_CAP_RRSBA)) {
> > +		rrsba_disabled = true;
> > +		return;
> > +	}
> 
> You'll take this path if you have out-of-date microcode.
> 
> RRSBA is only enumerated from September last year, IIRC.  (Definitely
> from this point on some CPUs.)
> 
> When RRSBA was introduced, I was under the (false) impression that all
> eIBRS systems suffered RRSBA, but it turns out that select parts
> (ICX,TGL,RKL) are non-RRSBA despite not having RRSBA_CTRL.

Hm, so the original code here had this problem too, right?

	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE) &&
	    !(x86_read_arch_cap_msr() & ARCH_CAP_RRSBA))
		return;

At this point I'm having a hard time caring about 7 months out-of-date
microcode, but is there a reasonable way to check for that?

-- 
Josh