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

[J. Mayer]

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.

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

It's used by PowerPC. You're right, this should be fixed too. I did not
add this fix because it has less impact than the endian stuff, at least
on PowerPC were you have explicit load/store with reverse-endian
instruction which perform poorly with the current implementation and are
well optimized using the reverse-endian patch: the current
implementation does 2 bytes-swaps when loading little-endian data from
memory on a little-endian machine instead of ... nothing.

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

softmmu_reverse_endian.diff
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