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Subject: Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages
To:     Peter Zijlstra <peterz@infradead.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     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
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>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <0bb024ce-11aa-80dc-c7d8-d5acc5329f25@csgroup.eu>
Date:   Wed, 17 Jun 2020 16:45:19 +0200
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Le 17/06/2020 à 16:38, Peter Zijlstra a écrit :
> 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?
> 

This should work as well. Indeed nobody cares about what's in the other 
three. They are only there to ensure that ptep++ increases the ptep 
pointer by 16 bytes. Only the HW require 4 identical values, that's 
taken care of in set_pte_at() and pte_update().
So we should use the most efficient. Thinking once more, maybe what you 
propose is the most efficient as there is no need to load another 
register with value 0 in order to write it in the stack.

Christophe