linux-kernel - Re: [patch V2 2/5] x86/kaiser: Simplify disabling of global pages

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Date:   Mon, 27 Nov 2017 10:15:33 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...hat.com>, daniel.gruss@...k.tugraz.at,
        hughd@...gle.com, keescook@...gle.com, linux-mm@...ck.org,
        michael.schwarz@...k.tugraz.at, moritz.lipp@...k.tugraz.at,
        richard.fellner@...dent.tugraz.at
Subject: Re: [patch V2 2/5] x86/kaiser: Simplify disabling of global pages

On 11/26/2017 03:14 PM, Thomas Gleixner wrote:
> +static void enable_global_pages(void)
> +{
> +#ifndef CONFIG_KAISER
> +	__supported_pte_mask |= _PAGE_GLOBAL;
> +#endif
> +}
> +
>  static void __init probe_page_size_mask(void)
>  {
>  	/*
> @@ -179,11 +186,11 @@ static void __init probe_page_size_mask(
>  		cr4_set_bits_and_update_boot(X86_CR4_PSE);
>  
>  	/* Enable PGE if available */
> +	__supported_pte_mask |= _PAGE_GLOBAL;
>  	if (boot_cpu_has(X86_FEATURE_PGE)) {
>  		cr4_set_bits_and_update_boot(X86_CR4_PGE);
> -		__supported_pte_mask |= _PAGE_GLOBAL;
> -	} else
> -		__supported_pte_mask &= ~_PAGE_GLOBAL;
> +		enable_global_pages();
> +	}

This looks a little funky.  Doesn't this or _PAGE_GLOBAL into
__supported_pte_mask twice?  Once before the if(), and then a second
time via enable_global_pages() inside the if?

Did you intend for it to be masked *out* in the first one and then or'd
back in via enable_global_pages()?