Re: KVM and variable-endianness guest CPUs

Christoffer Dall
2014-01-18T04:24:01Z

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.

--
Christoffer