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

[Cornelia Huck]

On Mon, 3 May 2021 11:17:54 +0200
David Hildenbrand <david@redhat.com> wrote:

> On 03.05.21 11:14, Cornelia Huck wrote:
> > On Mon, 03 May 2021 10:23:20 +0200
> > Markus Armbruster <armbru@redhat.com> wrote:
> >   
> >> 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?  
> > 
> > I fear the experience will be as bad as for any guest that is using
> > features from a newer cpu level (i.e. random crashes when the guest
> > actually tries to use that newer instruction.)
> > 
> > I see two options:
> > - Just try to start and hope that it works.
> > - Deprecate any cpu model older than z10.
> > 
> > Anyone have a better idea? I don't particularly like any of the two.  
> 
> As the default CPU model with new compat machines is >= z13, I wouldn't 
> lose sleep about this. Even with a broken bios one can still boot an 
> external kernel+initrd for testing purposes.

Yes, I do not see many people running into this problem. Still, I fear
it will be hard to figure out what exactly the problem is, when it
arises...