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

[Blue Swirl]

On 10/13/07, J. Mayer <address@hidden> wrote:
> On Sat, 2007-10-13 at 13:47 +0300, Blue Swirl wrote:
> > On 10/13/07, J. Mayer <address@hidden> 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.
>
> I realized that I/O accesses for reverse-endian pages were not correct
> in the softmmu_template.h header. This new version fixes this. It also
> remove duplicated code in the case of unaligned accesses in a
> reverse-endian page.

I think 64 bit access case is not handled correctly, but to solve that
it would be nice to extend the current IO access system to 64 bits.