Re: [PATCH 4/4] pc-bios/s390-ccw: Allow building with Clang, too

[Thomas Huth]

On 03/05/2021 06.58, Markus Armbruster wrote:
> Thomas Huth <thuth@redhat.com> writes:
>
> > Clang unfortunately does not support generating code for the z900
> > architecture level and starts with the z10 instead. Thus to be able
> > to support compiling with Clang, we have to check for the supported
> > compiler flags. The disadvantage is of course that the bios image
> > will only run with z10 guest CPUs upwards (which is what most people
> > use anyway), so just in case let's also emit a warning in that case.
>
> What happens when you try to use this bios with an old CPU anyway?

Interesting question. I was expecting the guest to crash since it would be 
using a CPU instruction that is not supported on the old CPU model. But I 
just gave it a try, and there was no crash. The guest booted just fine. 
Either Clang only emits instructions that work with the old z900 anyway, or 
our emulation in QEMU is imprecise and we allow newer instructions to be 
executed on old models, too.

 Thomas