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Subject: Re: [PATCH 07/16] x86/mm: Move two more functions from pgtable_64.h
 to pgtable.h
To: Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>
References: <1516120619-1159-1-git-send-email-joro@8bytes.org>
 <1516120619-1159-8-git-send-email-joro@8bytes.org>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
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From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <727a7eba-41a0-d5bb-df54-8e58b33fde76@intel.com>
Date: Tue, 16 Jan 2018 10:03:09 -0800
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On 01/16/2018 08:36 AM, Joerg Roedel wrote:
> +/*
> + * Page table pages are page-aligned.  The lower half of the top
> + * level is used for userspace and the top half for the kernel.
> + *
> + * Returns true for parts of the PGD that map userspace and
> + * false for the parts that map the kernel.
> + */
> +static inline bool pgdp_maps_userspace(void *__ptr)
> +{
> +	unsigned long ptr = (unsigned long)__ptr;
> +
> +	return (((ptr & ~PAGE_MASK) / sizeof(pgd_t)) < KERNEL_PGD_BOUNDARY);
> +}

One of the reasons to implement it the other way:

-	return (ptr & ~PAGE_MASK) < (PAGE_SIZE / 2);

is that the compiler can do this all quickly.  KERNEL_PGD_BOUNDARY
depends on PAGE_OFFSET which depends on a variable.  IOW, the compiler
can't do it.

How much worse is the code that this generates?