Re: Compiling the s390-ccw bios with clang (was: Re: s390-ccw: warning: writing 1 byte into a region of size 0)

[Thomas Huth]

On 23/04/2021 10.07, Cornelia Huck wrote:
> On Fri, 23 Apr 2021 09:57:08 +0200
> Thomas Huth <thuth@redhat.com> wrote:
>
> > On 23/04/2021 08.52, Christian Borntraeger wrote:
> > [...]
> > > We can of course discuss if we compile the BIOS for z10 instead of z900. TCG
> > > in the mean time can handle up to z13 and z10 is now also 13 years old.
> >
> > I'd really like to see us supporting Clang in the s390-ccw bios, too, since
> > it provides additional useful compiler warnings ... but switching the -mz900
> > to -mz10 here also means that we could not boot VMs anymore that use a CPU
> > that is older than the z10...
>
> We could still boot a kernel/initrd directly, couldn't we?

Yes, but that will certainly require some documentation effort to make it 
clear to the users that they need to use "-kernel" in case they want to run 
an older guest...

> > Is anybody still using such old CPUs? Should we maybe deprecate all CPUs
> > that are older than the z10 in QEMU? Alternatively, we could try to detect
> > Clang in the Makefile, and only use -mz10 in that case and continue to use
> > -mz900 in the other case...?
>
> So, the issue with clang is that it compiles to at least a z10, right?

Right, Clang does not support anything that is older than a z10.

> Any other issues we know about?

There are some compiler warnings, and at least network booting seems to be 
broken... that certainly needs some investigation first...

 Thomas