Re: [Qemu-devel] [PATCH 1/7] virtio: allow byte swapping for vring and config access

[Peter Maydell]

On 8 August 2013 17:07, Anthony Liguori <address@hidden> wrote:
> It's the same processor.  It still starts executing big endian
> instructions.  A magic register value is tweaked and loads/stores are
> swapped.

I dunno about PPC, but for ARM generally the boot-up state is
controlled by config signals which a SoC or board can hardwire,
so you can have a SoC which is configured to start in big-endian
mode.

> CPU data structures are still read as big endian though.

Do you have an example of what you mean by "CPU data structure"?

> The distinction is important in QEMU.  ppc64 is still
> TARGET_WORDS_BIGENDIAN.

Ideally TARGET_WORDS_BIGENDIAN would go away -- it is forcing
at compile time a setting which is actually a runtime one,
and a lot of the weirdness here flows from that.

> We still want most stl_phys to treat integers
> as big endian.

Any stl_phys() should [in an ideal design] be tied to a
"bus master" which has its own idea of which endianness
it is. That is, an stl_phys() for a DMA controller model
ought to use the endianness programmed for the DMA controller,
not whatever the CPU happens to be using.

-- PMM