Re: KVM and variable-endianness guest CPUs

Alexander Graf
2014-01-18T07:32:47Z

> Am 18.01.2014 um 05:24 schrieb Christoffer Dall <christoffer.dall@linaro.org>:
>
>> On Fri, Jan 17, 2014 at 06:52:57PM +0000, Peter Maydell wrote:
>>> On 17 January 2014 17:53, Peter Maydell <peter.maydell@linaro.org> 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