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

[Daniel P . Berrangé]

On Fri, Apr 23, 2021 at 10:22:28AM +0200, Thomas Huth wrote:
> 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.

IIUC, according to wikipedia

   - z10 series was introduced in 2008
   - z900 series was introduced in 2000

Even the z10 is well older than the oldest OS platform we support.

Though I presume people keep mainframes deployed for longer than commodity
x86 hardware, it doesn't seem too unreasonable to say z10 is the oldest
we'll support.

IIUC, downstreams like RHEL already require even newer hadware than z10


Regards,
Daniel
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