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 <berra...@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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
