Re: [PATCH 0/5] Reinstate ability to use Qemu on pre-SSE4.1 x86 hosts

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jun 12, 2024 at 01:21:26PM +0100, Daniel P. Berrangé wrote:
> On Wed, Jun 12, 2024 at 01:51:31PM +0200, Paolo Bonzini wrote:
> > On Wed, Jun 12, 2024 at 1:38 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
> > > This isn't anything to do with the distro installer. The use case is that
> > > the distro wants all its software to be able to run on the x86_64 baseline
> > > it has chosen to build with.
> > 
> > Sure, and they can patch the packages if their wish is not shared by
> > upstream. Alternatively they can live with the fact that not all users
> > will be able to use all packages, which is probably already the case.
> 
> Yep, there's almost certainly scientific packages that have done
> optimizations in their builds. QEMU is slightly more special
> though because it is classed as a "critical path" package for
> the distro. Even the QEMU linux-user pieces are now critical path,
> since they're leveraged by docker & podman for running foreign arch
> containers.
> 
> > Or drop QEMU, I guess. Has FeSCO ever expressed how strict they are
> > and which of the three options they'd pick?
> 
> I don't know - i'm going to raise this question to find out if
> there's any guidance.

I learnt that FESCo approved a surprisingly loose rule saying

  "Libraries packaged in Fedora may require ISA extensions,
   however any packaged application must not crash on any
   officially supported architecture, either by providing
   a generic fallback implementation OR by cleanly exiting
   when the requisite hardware support is unavailable."

This might suggest we could put a runtime feature check in main(),
print a warning and then exit(1), however, QEMU has alot of code
that is triggered from ELF constructors. If we're building the
entire of QEMU codebase with extra features enabled, I worry that
the constructors could potentially cause a illegal instruction
crash before main() runs ?

With regards,
Daniel
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