Re: [PATCH v7 10/10] x86/vmscape: Add cmdline vmscape=on to override attack vector controls

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

On Thu, Mar 19, 2026 at 04:40:21PM +0000, bot+bpf-ci@kernel.org wrote:
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -8375,9 +8375,11 @@
> >
> >  			off		- disable the mitigation
> >  			ibpb		- use Indirect Branch Prediction Barrier
> > -					  (IBPB) mitigation (default)
> > +					  (IBPB) mitigation
> >  			force		- force vulnerability detection even on
> >  					  unaffected processors
> > +			on		- (default) selects IBPB or BHB clear
> > +					  mitigation based on CPU
> 
> Is the "(default)" label accurate here?  The code default is
> VMSCAPE_MITIGATION_AUTO, not VMSCAPE_MITIGATION_ON:
> 
>     static enum vmscape_mitigations vmscape_mitigation __ro_after_init =
>         IS_ENABLED(CONFIG_MITIGATION_VMSCAPE) ?
>             VMSCAPE_MITIGATION_AUTO : VMSCAPE_MITIGATION_NONE;
> 
> In vmscape_select_mitigation(), AUTO and ON take the same switch
> path for selecting between BHB clear and IBPB.  But AUTO goes
> through this check first:
> 
>     if ((vmscape_mitigation == VMSCAPE_MITIGATION_AUTO) &&
>         !should_mitigate_vuln(X86_BUG_VMSCAPE))
>         vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
> 
> ON skips that check, which is the whole point of this commit per
> the subject line ("override attack vector controls").
> 
> A sysadmin reading "on (default)" would conclude that not passing
> any option gives the same behavior as vmscape=on, but the actual
> default respects attack vector controls while vmscape=on overrides
> them.

Thats a valid point. Updating the documentation as below:

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 1068569be5cf..98204d464477 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -8381,8 +8381,10 @@ Kernel parameters
 					  (IBPB) mitigation
 			force		- force vulnerability detection even on
 					  unaffected processors
-			on		- (default) selects IBPB or BHB clear
+			auto		- (default) use IBPB or BHB clear
 					  mitigation based on CPU
+			on		- same as "auto", but override attack
+					  vector control
 
 	vsyscall=	[X86-64,EARLY]
 			Controls the behavior of vsyscalls (i.e. calls to