Re: [PATCH v3 05/35] x86/bugs: Restructure taa mitigation

[Josh Poimboeuf]

On Tue, Feb 11, 2025 at 05:17:15PM +0000, Kaplan, David wrote:
> > On Wed, Jan 08, 2025 at 02:24:45PM -0600, David Kaplan wrote:
> > > @@ -400,48 +402,71 @@ static void __init taa_select_mitigation(void)
> > >               return;
> > >       }
> > >
> > > -     if (cpu_mitigations_off()) {
> > > +     if (cpu_mitigations_off())
> > >               taa_mitigation = TAA_MITIGATION_OFF;
> > > -             return;
> > > -     }
> > >
> > >       /*
> > >        * TAA mitigation via VERW is turned off if both
> > >        * tsx_async_abort=off and mds=off are specified.
> > > +      *
> > > +      * MDS mitigation will be checked in taa_update_mitigation().
> > >        */
> > > -     if (taa_mitigation == TAA_MITIGATION_OFF &&
> > > -         mds_mitigation == MDS_MITIGATION_OFF)
> > > +     if (taa_mitigation == TAA_MITIGATION_OFF)
> > >               return;
> >
> > This check seems rather pointless, the only thing after this is the
> > TAA_MITIGATION_AUTO check.
> 
> True, and it can be removed.  But in patch 4 in the mds logic you did suggest having an explicit return to make it clear that none of the later conditions applied.  I'm not sure I feel strongly either way, but I'd like to be consistent.

Let me try to clarify:

  - If it's already doing the conditional for another reason, then
    adding in the return makes sense:

	if (condition) {
	    	do_something;
		/* all actions related to condition are done */
		return;
	}

  - Or, if it's adding a condition+return to avoid having to explicitly
    check for !condition later, that also makes sense.

    	if (condition)
    		return;

    	/* assume !condition */
    	...

  - But adding condition+return, when there's only one condition
    remaining in the function, which already implicitly excludes the
    original condition, that just adds code for no reason.

	/* this has no purpose */
	if (condition)
		return;

	if (!condition && condition2)
		do_something;
    	
    return;

-- 
Josh