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

[Peter Maydell]

On Mon, 19 Dec 2022 at 06:35, David Gibson <david@gibson.dropbear.id.au> wrote:
>
> On Fri, Dec 16, 2022 at 09:39:19PM +0000, Peter Maydell wrote:
> > On Fri, 16 Dec 2022 at 19:11, Daniel Henrique Barboza
> > <danielhb413@gmail.com> wrote:
> > >
> > >
> > >
> > > On 12/13/22 10:51, Peter Maydell wrote:
> > > Yes, most if not all accesses are being handled as "target endian", even
> > > though the target is always big endian.
>
> So "target is always big endian" is pretty misleading for POWER.  We
> always define "TARGET_BIG_ENDIAN" in qemu, but for at least 10 years
> the CPUs have been capable of running in either big endian or little
> endian mode (selected at runtime).  Some variants can choose
> endianness on a per-page basis.  Since the creation of the ISA it's
> had "byte reversed" load and store instructions that let it use little
> endian for specific memory accesses.

Yeah, this is like Arm (and for the purposes of this thread
I meant essentially "TARGET_BIG_ENDIAN is always defined").

> Really the whole notion of an ISA having an "endianness" doesn't make
> a lot of sense - it's an individual load or store to memory that has
> an endianness which can depend on a bunch of factors.  When these
> macros were created, an ISA nearly always used the same endianness,
> but that's not really true any more - not just for POWER, but for a
> bunch of targets.  So from that point of view, I think getting rid of
> tswap() - particularly one that has compile time semantics, rather
> than behaviour which can depend on cpu mode/state is a good idea.

I tend to think of the TARGET_BIG_ENDIAN/not setting as being
something like "CPU bus endianness". At least for Arm, when you
put the CPU into BE mode it pretty much means "the CPU byteswaps
the data when it comes in/out", AIUI.

> I believe that even when running in little-endian mode, the hash page
> tables are encoded in big-endian, so I think the proposed change makes
> sense.

OK. I still think we should consistently change all the places that are
accessing this data structure, though, not just half of them.

thanks
-- PMM