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

[Anthony Liguori <anthony@codemonkey.ws>]

"Daniel P. Berrange" <berrange@redhat.com> writes:

> On Thu, Aug 08, 2013 at 10:40:28AM -0500, Anthony Liguori wrote:
>> Andreas Färber <afaerber@suse.de> writes:
>> >> We have a mechanism to do weak functions via stubs/.  I think it would
>> >> be better to do cpu_get_byteswap() as a stub function and then overload
>> >> it in the ppc64 code.
>> >
>> > If this as your name indicates is a per-CPU function then it should go
>> > into CPUClass. Interesting question is, what is virtio supposed to do if
>> > we have two ppc CPUs, one is Big Endian, the other is Little Endian.
>> 
>> PPC64 is big endian.  AFAIK, there is no such thing as a little endian
>> PPC64 processor.
>
> Unless I'm misunderstanding, this thread seems to suggest otherwise:
>
>   "[Qemu-devel] [PATCH 0/5] 64bit PowerPC little endian support"
>
>   https://lists.nongnu.org/archive/html/qemu-devel/2013-08/msg00813.html

Yeah, it's confusing.  It feels like little endian to most software but
the distinction in hardware (and therefore QEMU) is important.

It's the same processor.  It still starts executing big endian
instructions.  A magic register value is tweaked and loads/stores are
swapped.  CPU data structures are still read as big endian though.  It's
really just load/stores that are affected.

The distinction is important in QEMU.  ppc64 is still
TARGET_WORDS_BIGENDIAN.  We still want most stl_phys to treat integers
as big endian.  There's just this extra concept that CPU loads/stores
are sometimes byte swapped.  That affects virtio but not a lot else.

Regards,

Anthony Liguori

>
>
> Daniel
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