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

[Alexander Graf]

> Am 18.01.2014 um 05:24 schrieb Christoffer Dall <address@hidden>:
> 
>> On Fri, Jan 17, 2014 at 06:52:57PM +0000, Peter Maydell wrote:
>>> 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]
> 
> I completely agree, given that it's too late to be set on always LE/BE,
> I think the natural choice is something that allows a user to cast the
> byte array to an appropriate pointer type and dereference it.
> 
> And I think we need to amend the KVM API docs to specify this.

I don't see the problem. For ppc we always do mmio emulation as if the cpu was 
big endian. We've had an is_bigendian variable for that since the very first 
versions.


Alex

> 
> -- 
> Christoffer