On Mon, 3 May 2021 11:17:54 +0200 David Hildenbrand <da...@redhat.com> wrote:
> On 03.05.21 11:14, Cornelia Huck wrote: > > On Mon, 03 May 2021 10:23:20 +0200 > > Markus Armbruster <arm...@redhat.com> wrote: > > > >> David Hildenbrand <da...@redhat.com> writes: > >> > >>> On 03.05.21 07:17, Thomas Huth wrote: > >>>> On 03/05/2021 06.58, Markus Armbruster wrote: > >>>>> Thomas Huth <th...@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...
