linux-kernel - Re: [patch 2/2] x86/alternatives: Disable interrupts and sync when optimizing NOPs in place

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Message-ID: <20231208132204.GBZXMYfLSaIbn3BvN3@fat_crate.local>
Date:   Fri, 8 Dec 2023 14:22:04 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, paul.gortmaker@...driver.com,
        x86@...nel.org, regressions@...mhuis.info,
        richard.purdie@...uxfoundation.org, regressions@...ts.linux.dev
Subject: Re: [patch 2/2] x86/alternatives: Disable interrupts and sync when
 optimizing NOPs in place

On Thu, Dec 07, 2023 at 08:49:26PM +0100, Thomas Gleixner wrote:
> +static void __init_or_module noinline optimize_nops_inplace(u8 *instr, size_t len)
> +{
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
> +	optimize_nops(instr, len);
> +	sync_core();
> +	local_irq_restore(flags);
> +}
> +
>  /*
>   * In this context, "source" is where the instructions are placed in the
>   * section .altinstr_replacement, for example during kernel build by the
> @@ -438,7 +448,7 @@ void __init_or_module noinline apply_alt
>  		 *   patch if feature is *NOT* present.
>  		 */
>  		if (!boot_cpu_has(a->cpuid) == !(a->flags & ALT_FLAG_NOT)) {
> -			optimize_nops(instr, a->instrlen);
> +			optimize_nops_inplace(instr, a->instrlen);
>  			continue;
>  		}

Arguably, the proper thing to do here would be to convert the NOP
optimizing to the same 2-stage process as normal patching: write insns
into a buffer and text_poke* it.

VS what we currently do: operating straight on kernel memory.

Lemme put it on the TODO and see how ugly it becomes.

-- 
Regards/Gruss,
    Boris.

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