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

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Message-ID: <20180116191105.GC28161@8bytes.org>
Date:   Tue, 16 Jan 2018 20:11:05 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, 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 Tue, Jan 16, 2018 at 10:03:09AM -0800, Dave Hansen wrote:
> On 01/16/2018 08:36 AM, Joerg Roedel wrote:
> > +	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?

I havn't looked at the actual code this generates, but the
(PAGE_SIZE / 2) comparison doesn't work on 32 bit where the address
space is not always evenly split. I'll look into a better way to check
this.

Thanks,

	Joerg