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

[Thiemo Seufer <ths@networkno.de>]

J. Mayer wrote:
> On Sat, 2007-10-13 at 16:17 +0200, J. Mayer wrote:
> > On Sat, 2007-10-13 at 16:07 +0300, Blue Swirl wrote:
> > > On 10/13/07, J. Mayer <l_indien@magic.fr> wrote:
> > > > On Sat, 2007-10-13 at 13:47 +0300, 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.
> > > >
> > > > 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.
> > 
> > I think that if it was previously correct, it should still be, but... I
> > don't know how much having 64 bits I/O accesses is interresting, as I
> > don't know if there are real hw buses that have 64 bits data path...
> > 
> > Here's another version taking care of your remark about ldl memory
> > accessors.
> > * I replaced all ldl occurences with ldul
> > * when TARGET_LONG_BITS == 64, I also added ldsl accessors. And I
> > started using it in the PowerPC memory access micro-ops.
> > Then the patch is really more invasive than the previous ones.
> > This still does not break PowerPC or i386 target, as it seems.
> 
> Here's a new version. The only change is that, for consistency, I did
> add the big-endian and little-endian accessors that were documented in
> cpu-all.h as unimplemented. The implementation is quite trivial, having
> native and reverse-endian accessors available, and changes functionnally
> nothing to the previous version.

The previous version works fine here, FWIW.


Thiemo