Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages

[Peter Zijlstra]

On Thu, Jun 18, 2020 at 12:21:22AM +1000, Michael Ellerman wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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?