Messages in this thread | | | Subject | Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages | From | Christophe Leroy <> | 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
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