linux-kernel - Re: [PATCH 3/3] powerpc/8xx: Provide ptep

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Date:   Wed, 17 Jun 2020 16:45:19 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Peter Zijlstra <peterz@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages



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@...radead.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