linux-kernel - Re: [PATCH 07/16] x86/mm: Move two more functions from pgtable

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Message-ID: <727a7eba-41a0-d5bb-df54-8e58b33fde76@intel.com>
Date:   Tue, 16 Jan 2018 10:03:09 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>, aliguori@...zon.com,
        daniel.gruss@...k.tugraz.at, hughd@...gle.com, keescook@...gle.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, jroedel@...e.de
Subject: Re: [PATCH 07/16] x86/mm: Move two more functions from pgtable_64.h
 to pgtable.h

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?