Re: [Qemu-ppc] KVM and variable-endianness guest CPUs

[Peter Maydell]

On 17 January 2014 17:53, Peter Maydell <address@hidden> wrote:
> Specifically, the KVM API says "here's a uint8_t[] byte
> array and a length", and the current QEMU code treats that
> as "this is a byte array written as if the guest CPU
> (a) were in TARGET_WORDS_BIGENDIAN order and (b) wrote its
> I/O access to this buffer rather than to the device".
>
> The KVM API docs don't actually specify the endianness
> semantics of the byte array, but I think that that really
> needs to be nailed down. I can think of a couple of options:
>  * always LE
>  * always BE
>    [these first two are non-starters because they would
>    break either x86 or PPC existing code]
>  * always the endianness the guest is at the time
>  * always some arbitrary endianness based purely on the
>    endianness the KVM implementation used historically
>  * always the endianness of the host QEMU binary
>  * something else?
>
> Any preferences? Current QEMU code basically assumes
> "always the endianness of TARGET_WORDS_BIGENDIAN",
> which is pretty random.

Having thought a little more about this, my opinion is:

 * we should specify that the byte order of the mmio.data
   array is host kernel endianness (ie same endianness
   as the QEMU process itself) [this is what it actually
   is, I think, for all the cases that work today]
 * we should fix the code path in QEMU for handling
   mmio.data which currently has the implicit assumption
   that when using KVM TARGET_WORDS_BIGENDIAN is the same
   as the QEMU host process endianness (because it's using
   load/store functions which swap if TARGET_WORDS_BIGENDIAN
   is different from HOST_WORDS_BIGENDIAN)

thanks
-- PMM