Re: [PATCH 3/3] x86/alternative: Patch a single alternative location only once

[Peter Zijlstra]

On Tue, Sep 30, 2025 at 09:33:25AM +0200, Jürgen Groß wrote:
> On 30.09.25 09:26, Peter Zijlstra wrote:

> > > @@ -692,14 +698,19 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
> > >   		 * - feature not present but ALT_FLAG_NOT is set to mean,
> > >   		 *   patch if feature is *NOT* present.
> > >   		 */
> > > -		if (!boot_cpu_has(a->cpuid) == !(a->flags & ALT_FLAG_NOT)) {
> > > -			memcpy(insn_buff, instr, a->instrlen);
> > > -			optimize_nops(instr, insn_buff, a->instrlen);
> > > -		} else {
> > > +		if (!boot_cpu_has(a->cpuid) != !(a->flags & ALT_FLAG_NOT)) {
> > >   			apply_one_alternative(instr, insn_buff, a);
> > > +			patched = true;
> > >   		}
> > > -		text_poke_early(instr, insn_buff, a->instrlen);
> > > +		instances--;
> > > +		if (!instances) {
> > > +			if (!patched) {
> > > +				memcpy(insn_buff, instr, a->instrlen);
> > > +				optimize_nops(instr, insn_buff, a->instrlen);
> > > +			}
> > > +			text_poke_early(instr, insn_buff, a->instrlen);
> > > +		}
> > >   	}
> > >   	kasan_enable_current();
> > 
> > I think you lost the optimize_nops() call for the patched case.
> > 
> > That is, note how apply_one_alternative() does 0x90 padding, but then
> > you only do optimize_nops() when !patched.
> 
> The call of optimize_nops() is part of text_poke_apply_relocation() when
> patching, like without my series.

Indeed it is. Clearly I need more wakeup juice :-)

Therefore the patches seem fine. I'll try and hold onto them until the
merge window closes and then stick them in x86/core or thereabout.