Re: [Qemu-devel] qemu hw/ppc_oldworld.c target-ppc/cpu.h target-...

[Thiemo Seufer]

Paul Brook wrote:
> > > I think what you mean is that they work the way that ppc64 is defined, to
> > > remain compatible with ppc32.  IMHO this is entirely irrelevant as we're
> > > emulating a ppc32. You could replace the high bits with garbage and
> > > nothing would ever be able to tell the difference.
> >
> > PowerPC is a 64 bits architecture. PowerPC 32 on 32 bits host is
> > optimized not to compute the 32 highest bits, the same way it's allowed
> > to cut down the GPR when implementing a CPU that would not support the
> > 64 bits mode (but this is a tolerance, this is not the architecture is
> > defined).
> 
> No. PowerPC is defined as a 64-bit archirecure. However there is a subset of 
> this architecture (aka ppc32) that is a complete 32-bit architecture in its 
> own right.  By your own admission, we can get away with not calculating the 
> high 32 bit of the register. If follows that the high bits are completely 
> meaningless. 

This btw. also means that the ppc32 emulation on 32-bit hosts is needlessly
inefficient if the high bits are carried around.

> The qemu ppc32 emulation is implemented in such a way that on 64-bit hosts it 
> looks a lot like a ppc64 implementation. However this need not, and should 
> not be exposed to the user.
> 
> > OK. Those are real bugs to be fixed. I'll take a look.... But I'll try
> > not to break the GPR dump. In fact, GPR should always dumped as 64 bits,
> > even when runnig on 32 bits hosts. This would be more consistent with
> > the specification.
> 
> I disagree. qemu is implementing ppc32. Showing more than 32 bits of register 
> is completely bogus. Any differences between a 32-bit host and a 64-bit host 
> are a qemu bug. If you display 64 bits, then those 64 bits had better be the 
> same when run on 32-bit hosts.

I strongly agree with Paul.


Thiemo