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


From: J. Mayer
Subject: Re: [Qemu-devel] RFC: reverse-endian softmmu memory accessors
Date: Sun, 14 Oct 2007 00:07:27 +0200

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 <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.
> 
> 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.

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

Attachment: softmmu_reverse_endian.diff
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