Re: [Qemu-devel] Inquiry, speed comparison on OS X, QEMU vs Virtual PC

[Pierre d'Herbemont]

Le 10 juil. 04, à 15:08, J. Mayer a écrit :

> Le 9 juil. 04, à 22:07, Natalia Portillo a écrit :
> > > I think that including support for the PowerPC swapping instructions
> > > in QEMU
> > > will break compatibility with host PowerPCs before G3, so that
> > > instructions
> > > should be used in a run-time capability detection scheme.
>
> On Sat, 2004-07-10 at 14:47, Pierre d'Herbemont wrote:
> > Which PowerPC Implementations? According to The PowerPC Architecture
> > Book, all the implementation 601,603,604,620 should include support 
> > for
> > the lwbrx and stwbrx instructions.
>
> I think all PPC include those instructions and gcc already uses them 
> for
> endian-swapping.
> It seems to me that the feature which is discussed here is the presence
> of IE bit in the machine state register.

you mean the LE bit? I first read the IE bit ;) Again according the 
this same book the LE bit should exits in the previous implementation.

> I don't think using this bit could improve performances a lot. Because
> there is only one bit to control the CPU and because most of the
> problems are already solved using brx load and stores.
> You would need to patch the kernel to use this bit (if it exists) and
> need kernel calls when you ever want to change the access mode, which
> seems not so great.

ok, thanks... I wonder if it can help the darwine project in some ways: 
having Wine compiled as LE/PowerPC, and running it in some kind of 
virtualizer which would only swap sycall... Anyway I was speaking at 
the begining of the thread of the *brx instructions which are not used 
in the Mac OS X build.

> What could be more benefic would be a feature which exists in the PPC
> 405: one can define a memory area using big or little endian accesses.
> But this seems to be very specific to the 405 and, once again, the win
> would not be spectacular, imho.

In fact you can emulate this with mprotect-ing the region, and handle 
the conversion at signal catch, but it shouldn't be really performant.

Pierre