linux-kernel - Re: [PATCH 2/2] x86/mm: Introduce 'no5lvl' kernel parameter

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Date:   Sun, 13 May 2018 23:55:00 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
cc:     Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Hugh Dickins <hughd@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/mm: Introduce 'no5lvl' kernel parameter

On Fri, 11 May 2018, Kirill A. Shutemov wrote:
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1008,6 +1008,12 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>  	 */
>  	setup_clear_cpu_cap(X86_FEATURE_PCID);
>  #endif
> +
> +#ifdef CONFIG_X86_5LEVEL
> +	/* Clear the 5-level paging feature if user asked for 'no5lvl' */

no5lvl is only one reason why 5 level paging is not available.

> +	if (!__pgtable_l5_enabled)
> +		setup_clear_cpu_cap(X86_FEATURE_LA57);
> +#endif

And that #ifdeffery can be avoided by simply doing:

	if (IS_ENABLED(CONFIG_X86_5LEVEL) && !pgtable_l5_enabled)
		setup_clear_cpu_cap(X86_FEATURE_LA57);

Hmm?

Thanks,

	tglx