Re: [Qemu-devel] RFC: reverse-endian softmmu memory accessors

[Thiemo Seufer <ths@networkno.de>]

Blue Swirl wrote:
> On 10/13/07, J. Mayer <l_indien@magic.fr> wrote:
> > The problem:
> > some CPU architectures, namely PowerPC and maybe others, offers
> > facilities to access the memory or I/O in the reverse endianness, ie
> > little-endian instead of big-endian for PowerPC, or provide instruction
> > to make memory accesses in the "reverse-endian". This is implemented as
> > a global flag on some CPU. This case is already handled by the PowerPC
> > emulation but is is far from being optimal. Some other implementations
> > allow the OS to store an "reverse-endian" flag in the TLB or the segment
> > descriptors, thus providing per-page or per-segment endianness control.
> > This is mostly used to ease driver migration from a PC platform to
> > PowerPC without taking any care of the device endianness in the driver
> > code (yes, this is bad...).
> 
> Nice, this may be useful for Sparc64. It has a global CPU flag for
> endianness, individual pages can be marked as reverse endian, and
> finally there are instructions that access memory in reverse endian.
> The end result is a XOR of all these reverses. Though I don't know if
> any of these features are used at all.

Likewise for the MIPS reverse endianness global flag.

> Other memory access functions could be merged too. Is the 32 bit load
> with sign extension to 64 bits used in other architectures?

Yes for MIPS.


Thiemo