Re: [Qemu-devel] [PATCH v2] qtest: ask endianness of the target in qtest_init()

[David Gibson]

On Tue, Oct 11, 2016 at 12:24:29PM +1100, David Gibson wrote:
> On Mon, Oct 10, 2016 at 03:10:33PM +0100, Peter Maydell wrote:
> > On 10 October 2016 at 14:39, David Gibson <address@hidden> wrote:
> > > In the overwhelming majority of cases the endianness of the device is
> > > known independent of the guest CPU and board.
> > 
> > We don't seem to model things that way:
> > 
> > $ git grep '.endianness = DEVICE_NATIVE' |wc -l
> > 341
> > $ git grep '.endianness = DEVICE_BIG' |wc -l
> > 18
> > $ git grep '.endianness = DEVICE_LITTLE' |wc -l
> > 172
> 
> Sigh.  So, most of these are themselves mistakes.
> 
> A lot of these are target specific devices that should be tagged with
> their target's (notional) endianness, rather than NATIVE.  That
> covers:

A couple of points of clarification here.

When I say the "device" endianness is known, that's a little sloppy.
I mean that the endianness of each specific register (or DMA field)
has a known endianness.  The device as a whole does not have well
defined endianness except insofar as one device will generally use the
same endianness for all its >8bit registers and/or DMA fields.

There are a few exceptions of course.  virtio-balloon (pre 1.0) on a
notionally BE platform is a horrible example: its PCI config space is
LE, the generic part of the virtio config space is BE ("native") and
the balloon specific part is LE again (due to a screwup in the spec).

Intermediate bridges in the system (on or off chip) won't affect this,
*as long as* they preserve byte address invariance.  If they *don't*
preserve byte address invariance, then:
        1) Shoot your HW designer
        2) Grudgingly work around with special cases in your tests


-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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