Re: [Qemu-devel] [Qemu-ppc] [PATCH qemu REPOST] spapr/rtas: Force big endian compile for rtas

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Jun 17, 2019 at 10:25:10AM +0200, Greg Kurz wrote:
65;5603;1c> On Wed, 12 Jun 2019 12:07:23 +1000
> Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
> 
> > At the moment the rtas's Makefile uses generic QEMU rules which means
> > that when QEMU is compiled on a little endian system, the spapr-rtas.bin
> > is compiled as little endian too which is incorrect as it is always
> > executed in big endian mode.
> 
> I'm naively thinking that executing code compiled as little endian
> in big endian mode would result in an exception... Can you explain
> how/why this ever worked ?

Because basically nobody actually built the rtas blob from the
sources, they just used the pre-compiled blob, which is correctly
built BE.

That said executing LE code in BE mode won't necessarily result in an
exception - it'll just execute whatever the instructions are you get
when you byte reverse the ones you inteded, which may or may not be
valid.  It's *likely* to cause an exception fairly soon, but the
opcode space is densely populated enough that there's a good chance it
won't cause an immediate illegal instruction.

-- 
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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