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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Peter Zijlstra <peterz@infradead.org>
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
In-Reply-To: <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> <20200617143826.GJ2531@hirez.programming.kicks-ass.net>
Date:   Thu, 18 Jun 2020 10:58:26 +1000
Message-ID: <87pn9xchql.fsf@mpe.ellerman.id.au>
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Peter Zijlstra <peterz@infradead.org> writes:
> 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?

I think val wants to be pte_basic_t, but otherwise yeah I like that much
better.

cheers