Re: [PATCH v5 11/16] hw/microblaze: Support various endianness for s3adsp1800 machines

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Feb 06, 2025 at 07:24:55PM +0100, Philippe Mathieu-Daudé wrote:
> +Michal
> 
> On 6/2/25 19:06, Daniel P. Berrangé wrote:
> > On Thu, Feb 06, 2025 at 06:49:38PM +0100, Philippe Mathieu-Daudé wrote:
> > > On 6/2/25 18:12, Daniel P. Berrangé wrote:
> > > > On Thu, Feb 06, 2025 at 04:04:20PM +0100, Philippe Mathieu-Daudé wrote:
> > > > > On 6/2/25 15:31, Daniel P. Berrangé wrote:
> > > > > > On Thu, Feb 06, 2025 at 02:53:58PM +0100, Philippe Mathieu-Daudé wrote:
> > > > > > > Hi Daniel,
> > > > > > > 
> > > > > > > On 6/2/25 14:20, Daniel P. Berrangé wrote:
> > > > > > > > On Thu, Feb 06, 2025 at 02:10:47PM +0100, Philippe Mathieu-Daudé wrote:
> > > > > > > > > Introduce an abstract machine parent class which defines
> > > > > > > > > the 'little_endian' property. Duplicate the current machine,
> > > > > > > > > which endian is tied to the binary endianness, to one big
> > > > > > > > > endian and a little endian machine; updating the machine
> > > > > > > > > description. Keep the current default machine for each binary.
> > > > > > > > > 
> > > > > > > > > 'petalogix-s3adsp1800' machine is aliased as:
> > > > > > > > > - 'petalogix-s3adsp1800-be' on big-endian binary,
> > > > > > > > > - 'petalogix-s3adsp1800-le' on little-endian one.
> > > > > > > > 
> > > > > > > > Does it makes sense to expose these as different machine types ?
> > > > > > > > 
> > > > > > > > If all the HW is identical in both cases, it feels like the
> > > > > > > > endianness could just be a bool property of the machine type,
> > > > > > > > rather than a new machine type.
> > > > > > > 
> > > > > > > Our test suites expect "qemu-system-foo -M bar" to work out of
> > > > > > > the box, we can not have non-default properties.
> > > > > > > 
> > > > > > > (This is related to the raspberry pi discussion in
> > > > > > > https://lore.kernel.org/qemu-devel/20250204002240.97830-1-philmd@linaro.org/).
> > > > > > > 
> > > > > > > My plan is to deprecate 'petalogix-s3adsp1800', so once we
> > > > > > > remove it we can merge both qemu-system-microblaze and
> > > > > > > qemu-system-microblazeel into a single binary.
> > > > > > > 
> > > > > > > If you don't want to add more machines, what should be the
> > > > > > > endianness of the 'petalogix-s3adsp1800' machine in a binary
> > > > > > > with no particular endianness? Either we add for explicit
> > > > > > > endianness (fixing test suites) or we add one machine for
> > > > > > > each endianness; I fail to see other options not too
> > > > > > > confusing for our users.
> > > > > > 
> > > > > > We would pick an arbitrary endianness of our choosing
> > > > > > I guess. How does this work in physical machines ? Is
> > > > > > the choice of endianess a firmware setting, or a choice
> > > > > > by the vendor when manufacturing in some way ?
> > > > > 
> > > > > Like MIPS*, SH4* and Xtensa*, it is a jumper on the board
> > > > > (wired to a CPU pin which is sampled once at cold reset).
> > > > 
> > > > That makes me thing even more it is just a machine type property.
> > > 
> > > I'm happy with a machine property, this was even my first approach
> > > using OnOffAuto until I ran the test-suite and have qom-introspection
> > > failing.
> > > 
> > > What should be the default?
> > > 
> > > Per the SH4 datasheet:
> > > 
> > >    Bit 31—Endian Flag (ENDIAN): Samples the value of the endian
> > >    specification external pin (MD5) in a power-on reset by the
> > >    RESET pin. The endian mode of all spaces is determined by this
> > >    bit. ENDIAN is a read-only bit.
> > > 
> > > There is no default per the spec, and I believe using one is
> > > a mistake.
> > 
> > If it is left as an unspecified choice in the spec, then I would
> > presume HW vendors are picking an option based on what they
> > expect "most" common usage to be amongst their customers. IOW,
> > if we know of typically used guest OS prefer big or little, that
> > could influence our choice.
> 
> Please have a look at this thread:
> https://lore.kernel.org/qemu-devel/d3346a55-584b-427b-8c87-32f7411a9290@amd.com/

That seems to give a pretty clear choice for qemu defaults

 "I am not aware about anybody configuring MB to big endian"

so in that particular case, defaulting to LE would be most sensible.

With regards,
Daniel
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