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

[J. Mayer]

On Sat, 2007-11-24 at 02:32 +0100, J. Mayer wrote:
> On Sat, 2007-11-24 at 00:52 +0000, Paul Brook wrote:
> > > >  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.
> > >
> > > Not completelly. There are even some way to do 64 bits computations when
> > > running in 32 bits mode... Some may see this as an architecture hack,
> > > but this gives the only way to switch from 32 bits to 64 bits mode (as
> > > the sixty-four MSR bits lies in the highest bits of the register).
> > 
> > Anything that involves switching to 64-bit mode to see th results is 
> > irelevant 
> > because we don't implement that.
> >
> > You can't have it both ways. Either you need to implement the full 64-bit 
> > gpr 
> > for correctness, in which case I guess we're most of the way to scrapping 
> > ppc-softmmu and using ppc64-softmmu all the time, or the high bits are not 
> > part of the interesting CPU state.
> 
> Yes, when running on a 64 bits host, we could avoid compiling
> ppc-softmmu. It's still interresting to use it on 32 bits host, as an
> optimisation, because it runs much faster than the ppc64-softmmu
> version.
> 
> > I can believe that on some hosts it's cheaper to use a 64-bit gpr_t, and 
> > the 
> > architecture/implementation is such that it gives the same results as a 
> > 32-bit gpr_t. However this is an implementation detail, and should not be 
> > exposed to the user.

I did forget one important point: it's not exposed to the qemu user.
The qemu log is a qemu developper tool.Most of the trace involved has to
be explicitally activated with specific defines in the Qemu code. The
qemu log is not to be of any help for a end-user, it's a tool for Qemu
development and bug reports.
>From the end-user side, the fact that GPR are implemented as 64 bits
values is never visible, even from the gdb stub. Then...

[...]

-- 
J. Mayer <address@hidden>
Never organized