Re: [Patch v3 03/13] x86/speculation: Add static key for Enhanced IBRS

[Thomas Gleixner]

On Fri, 26 Oct 2018, Tim Chen wrote:
> On 10/26/2018 09:58 AM, Waiman Long wrote:
> >> @@ -386,6 +389,7 @@ static void __init spectre_v2_select_mitigation(void)
> >>  			/* Force it so VMEXIT will restore correctly */
> >>  			x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
> >>  			wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
> >> +			static_branch_enable(&spectre_v2_enhanced_ibrs);
> >>  			goto specv2_set_mode;
> >>  		}
> >>  		if (IS_ENABLED(CONFIG_RETPOLINE))
> > 
> > Why you need a static key for enhanced IBRS? It is supposed to be set at
> > boot time and never get changed after that. It will be easier to use a
> > feature bit for that instead. We usually use static key when the value
> > can be changed at run time.
> >
> 
> We're close to running out of the feature bits.  So I'm trying not to
> use those.

Software feature bits can be extended when needed. That's really a non issue.

Thanks,

	tglx