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Date:   Wed, 17 Jun 2020 16:38:26 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-mm@kvack.org
Subject: Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages
Message-ID: <20200617143826.GJ2531@hirez.programming.kicks-ass.net>
References: <cover.1592225557.git.christophe.leroy@csgroup.eu>
 <341688399c1b102756046d19ea6ce39db1ae4742.1592225558.git.christophe.leroy@csgroup.eu>
 <20200615132244.GR2531@hirez.programming.kicks-ass.net>
 <87wo45db8d.fsf@mpe.ellerman.id.au>
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On Thu, Jun 18, 2020 at 12:21:22AM +1000, Michael Ellerman wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> > On Mon, Jun 15, 2020 at 12:57:59PM +0000, Christophe Leroy wrote:

> >> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES)
> >> +#define __HAVE_ARCH_PTEP_GET
> >> +static inline pte_t ptep_get(pte_t *ptep)
> >> +{
> >> +	pte_t pte = {READ_ONCE(ptep->pte), 0, 0, 0};
> >> +
> >> +	return pte;
> >> +}
> >> +#endif
> >
> > Would it make sense to have a comment with this magic? The casual reader
> > might wonder WTH just happened when he stumbles on this :-)
> 
> I tried writing a helpful comment but it's too late for my brain to form
> sensible sentences.
> 
> Christophe can you send a follow-up with a comment explaining it? In
> particular the zero entries stand out, it's kind of subtle that those
> entries are only populated with the right value when we write to the
> page table.

static inline pte_t ptep_get(pte_t *ptep)
{
	unsigned long val = READ_ONCE(ptep->pte);
	/* 16K pages have 4 identical value 4K entries */
	pte_t pte = {val, val, val, val);
	return pte;
}

Maybe something like that?