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

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Message-ID: <20180514215057.yodhn3nk37fnsxui@black.fi.intel.com>
Date:   Tue, 15 May 2018 00:50:57 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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 Sun, May 13, 2018 at 09:55:00PM +0000, Thomas Gleixner wrote:
> 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.

The 5-level paging may not be available for few reasons:
 - CONFIG_X86_5LEVEL=n
 - Machine doesn't support X86_FEATURE_LA57 -- clearing is nop;
 - User asked for 'no5lvl'.

To me the one-line comment reflects the situation reasonably well.

Do you want more verbose comment here?

> > +	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?

Unfortunately, no.

pgtable_l5_enabled is defined as cpu_feature_enabled(X86_FEATURE_LA57).
So with such change we would only clear the feature bit if it's already
clear.

-- 
 Kirill A. Shutemov