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

[Markus Armbruster]

David Hildenbrand <david@redhat.com> writes:

> On 03.05.21 07:17, Thomas Huth wrote:
>> 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.
>
> Yes, that's currently still done. We once thought about disabling that 
> (there was a patch from Richard), but decided against it because -- back 
> then -- the default QEMU model was still very basic and would have 
> essentially disabled all more recent instructions as default.
>
> We can most probably do that change soon as we have a "fairly new" 
> default QEMU CPU model. I can glue it to my z14 change.

In case this makes the BIOS crash with old CPUs: when a guest refuses to
start because the BIOS was compiled the wrong way for it, configure
having told you so back then is not a nice user experience.  Can we do
better, with reasonable effort?