linux-kernel - Re: [tip: x86/core] x86/alternative: Patch a single alternative location only once

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Message-ID: <20251014125909.GAaO5JHU_cgsPgstc_@fat_crate.local>
Date: Tue, 14 Oct 2025 14:59:09 +0200
From: Borislav Petkov <bp@...en8.de>
To: linux-kernel@...r.kernel.org
Cc: linux-tip-commits@...r.kernel.org, Juergen Gross <jgross@...e.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org
Subject: Re: [tip: x86/core] x86/alternative: Patch a single alternative
 location only once

On Tue, Oct 14, 2025 at 08:42:34AM -0000, tip-bot2 for Juergen Gross wrote:
> @@ -648,6 +648,8 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
>  	u8 insn_buff[MAX_PATCH_LEN];
>  	u8 *instr;
>  	struct alt_instr *a, *b;
> +	unsigned int instances = 0;
> +	bool patched = false;

Except that we have the reverse fir tree rule in tip for function-local vars.

The tip-tree preferred ordering of variable declarations at the
beginning of a function is reverse fir tree order::

	struct long_struct_name *descriptive_name;
	unsigned long foo, bar;
	unsigned int tmp;
	int ret;

The above is faster to parse than the reverse ordering::

	int ret;
	unsigned int tmp;
	unsigned long foo, bar;
	struct long_struct_name *descriptive_name;

And even more so than random ordering::

	unsigned long foo, bar;
	int ret;
	struct long_struct_name *descriptive_name;
	unsigned int tmp;

> @@ -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) {

I don't see how this is making this code better - this is slowly turning into
an unreadable mess with those magic "instances" and "patched".

And frankly, the justification for this patch is also meh: an interrupt might
use the location?!? If this is a real issue then we better disable IRQs around
it. But not make the code yucky.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette