Re: [RFC PATCH-for-8.0 2/3] hw/ppc/spapr: Replace tswap64(HPTE) by cpu_to_be64(HPTE)

[David Gibson]

On Wed, Dec 21, 2022 at 10:15:28PM +0000, Peter Maydell wrote:
> On Wed, 21 Dec 2022 at 16:03, Cédric Le Goater <clg@kaod.org> wrote:
> >
> > On 12/21/22 13:33, Peter Maydell wrote:
> > > On Wed, 21 Dec 2022 at 01:35, David Gibson <david@gibson.dropbear.id.au> 
> > > wrote:
> > >> On Mon, Dec 19, 2022 at 10:39:40AM +0000, Peter Maydell wrote:
> > >>> OK. I still think we should consistently change all the places that are
> > >>> accessing this data structure, though, not just half of them.
> > >>
> > >> Yes, that makes sense.  Although what exactly constitutes "this data
> > >> structure" is a bit complex here.  If we mean just the spapr specific
> > >> "external HPT", then there are only a few more references to it.  If
> > >> we mean all instances of a powerpc hashed page table, then there are a
> > >> bunch more in the cpu target code.
> > >
> > > I had in mind "places where we write this specific array of bytes
> > > spapr->htab".

Seems a reasonable amount to tackle for now.

> > spapr_store_hpte() seems to be the most annoying part. It is used
> > by hcalls h_enter, h_remove, h_protect. Reworking the interface
> > to present pte0/pte1 as BE variables means reworking the whole
> > hw/ppc/spapr_softmmu.c file. That's feasible but not a small task
> > since the changes will root down in the target hash mmu code which
> > is shared by all platforms ... :/
> 
> Don't you just need to change spapr_store_hpte() to use stq_be_p()
> instead of stq_p() ?

I think Peter is right.  The values passed to the function are "host
endian" (really, they don't have an endianness since they'll be in
registers).

> > spapr_hpte_set_c() are spapr_hpte_set_r() are of a different kind.
> 
> That code seems to suggest we already implicitly assume that
> spapr->htab fields have a given endianness...

Yes, we absolutely do.  We rely on the HPTE always being big-endian.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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