Date: Tue, 16 Jan 2018 20:34:59 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Joerg Roedel <joro@8bytes.org>
cc: Dave Hansen <dave.hansen@intel.com>,
        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
In-Reply-To: <20180116191105.GC28161@8bytes.org>
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References: <1516120619-1159-1-git-send-email-joro@8bytes.org> <1516120619-1159-8-git-send-email-joro@8bytes.org> <727a7eba-41a0-d5bb-df54-8e58b33fde76@intel.com> <20180116191105.GC28161@8bytes.org>
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Sender: linux-kernel-owner@vger.kernel.org

On Tue, 16 Jan 2018, Joerg Roedel wrote:

> 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.

It should be trivial enough to do

   return (ptr & ~PAGE_MASK) < PGD_SPLIT_SIZE);

and define it PAGE_SIZE/2 for 64bit and for PAE make it depend on the
configured address space split.

Thanks,

	tglx