Re: [PATCH] linux-user: use 'max' instead of 'qemu32' / 'qemu64' by defualt

[Daniel P . Berrangé]

On Fri, Aug 26, 2022 at 01:05:13PM +0100, Richard W.M. Jones wrote:
> On Fri, Aug 26, 2022 at 12:39:00PM +0100, Daniel P. Berrangé wrote:
> > The 'qemu64' CPU model implements the least featureful x86_64 CPU that's
> > possible. Historically this hasn't been an issue since it was rare for
> > OS distros to build with a higher mandatory CPU baseline.
> > 
> > With RHEL-9, however, the entire distro is built for the x86_64-v2 ABI
> > baseline:
> > 
> >   
> > https://developers.redhat.com/blog/2021/01/05/building-red-hat-enterprise-linux-9-for-the-x86-64-v2-microarchitecture-level
> > 
> > It is likely that other distros may take similar steps in the not too
> > distant future. For example, it has been suggested for Fedora on a
> > number of occassions.
> > 
> > This new baseline is not compatible with the qemu64 CPU model though.
> > While it is possible to pass a '-cpu xxx' flag to qemu-x86_64, the
> > usage of QEMU doesn't always allow for this. For example, the args
> > are typically controlled via binfmt rules that the user has no ability
> > to change. This impacts users who are trying to use podman on aarch64
> > platforms, to run containers with x86_64 content. There's no arg to
> > podman that can be used to change the qemu-x86_64 args, and a non-root
> > user of podman can not change binfmt rules without elevating privileges:
> > 
> >   https://github.com/containers/podman/issues/15456#issuecomment-1228210973
> > 
> > Changing to the 'max' CPU model gives 'qemu-x86_64' maximum
> > compatibility with binaries it is likely to encounter in the wild,
> > and not likely to have a significant downside for existing usage.
> > 
> > Most other architectures already use an 'any' CPU model, which is
> > often mapped to 'max' (or similar) already, rather than the oldest
> > possible CPU model.
> > 
> > For the sake of consistency the 'i386' architecture is also changed
> > from using 'qemu32' to 'max'.
> > 
> > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> > ---
> >  linux-user/i386/target_elf.h   | 2 +-
> >  linux-user/x86_64/target_elf.h | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/linux-user/i386/target_elf.h b/linux-user/i386/target_elf.h
> > index 1c6142e7da..238a9aba73 100644
> > --- a/linux-user/i386/target_elf.h
> > +++ b/linux-user/i386/target_elf.h
> > @@ -9,6 +9,6 @@
> >  #define I386_TARGET_ELF_H
> >  static inline const char *cpu_get_model(uint32_t eflags)
> >  {
> > -    return "qemu32";
> > +    return "max";
> >  }
> >  #endif
> > diff --git a/linux-user/x86_64/target_elf.h b/linux-user/x86_64/target_elf.h
> > index 7b76a90de8..3f628f8d66 100644
> > --- a/linux-user/x86_64/target_elf.h
> > +++ b/linux-user/x86_64/target_elf.h
> > @@ -9,6 +9,6 @@
> >  #define X86_64_TARGET_ELF_H
> >  static inline const char *cpu_get_model(uint32_t eflags)
> >  {
> > -    return "qemu64";
> > +    return "max";
> >  }
> >  #endif
> 
> Can we be assured we won't ever hit this TCG bug that currently
> affects -cpu max ?
> 
> https://gitlab.com/qemu-project/qemu/-/issues/1023
> 
> I'm going to guess we will be OK because qemu-user doesn't run a
> kernel and therefore wouldn't normally touch %cr3.  Is there any other
> situation?  (Of course it would be better all round if that glaring
> bug could be fixed.)

Yeah, the bug appears to be an interaction with the VM configuring
page tables, and since qemu-user is not doing that my guess it it
won't affect this usage. If we did want to be totally safe, we could
add -la57, since that feature flag is useless for user emulation
anyway.


With regards,
Daniel
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