Date: Tue, 16 Jan 2018 20:11:05 +0100
From: Joerg Roedel <joro@8bytes.org>
To: Dave Hansen <dave.hansen@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <jgross@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Brian Gerst <brgerst@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Eduardo Valentin <eduval@amazon.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Will Deacon <will.deacon@arm.com>, aliguori@amazon.com,
        daniel.gruss@iaik.tugraz.at, hughd@google.com, keescook@google.com,
        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <llong@redhat.com>, jroedel@suse.de
Subject: Re: [PATCH 07/16] x86/mm: Move two more functions from pgtable_64.h
 to pgtable.h
Message-ID: <20180116191105.GC28161@8bytes.org>
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 <727a7eba-41a0-d5bb-df54-8e58b33fde76@intel.com>
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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