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

[Peter Maydell]

On 8 August 2013 16:40, Anthony Liguori <address@hidden> wrote:
> PPC64 is big endian.  AFAIK, there is no such thing as a little endian
> PPC64 processor.

What's your definition of "little endian processor" here if
it isn't "one which is doing byte swaps of data"? I would
describe a PPC64 with the relevant mode bit set as "little
endian".

> This is just a processor mode where loads/stores are byte lane swapped.
> Hence the name 'cpu_get_byteswap'.  It's just asking whether the
> load/stores are being swapped or not.
>
> At least for PPC64, it's not possible to enable/disable byte lane
> swapping for individual CPUs.  It's done through a system-wide hcall.
>
> FWIW, I think most bi-endian architectures are this way too so I think
> this is equally applicable to ARM.  Peter, is that right?

ARM's bi-endian story is complicated and depends on the CPU.
Older CPUs didn't do byte-lane swapping of data; instead
when the CPU was configured in "big endian" mode they would
do address munging (XOR the address with 3 when doing a byte
access; XOR with 1 for halfword access). New CPUs do byte-lane
swapping (but only for data, not for instruction fetch).
CPUs in the transition period can do either.

In all cases, this is a per-cpu-core thing: you can have
one core configured to be bigendian and the other little
endian. You could in theory have the OS support both big
and little endian userspace processes. We ideally would
want to support "QEMU is a little endian process but the
VM's vcpu is currently bigendian".

Fuller writeup here:
http://translatedcode.wordpress.com/2012/04/02/this-end-up/

-- PMM