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

[Christophe Leroy]

Le 17/06/2020 Ã 16:38, Peter Zijlstra a ÃcritÂ:
> 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?

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