[PATCH v2] linux-user: use 'max' instead of 'qemu32' / 'qemu64' by default

[PATCH v2] linux-user: use 'max' instead of 'qemu32' / 'qemu64' by default

[Daniel P. Berrangé]

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 occasions.

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'.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---

Changed in v2:

 - Fixed commit message typos

 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
-- 
2.37.2

Re: [PATCH v2] linux-user: use 'max' instead of 'qemu32' / 'qemu64' by default

[Laurent Vivier]

Le 23/09/2022 à 13:04, Daniel P. Berrangé a écrit :
> 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 occasions.
> 
> 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'.
> 
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
> 
> Changed in v2:
> 
>   - Fixed commit message typos
> 
>   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

Applied to my linux-user-for-7.2 branch.

Thanks,
Laurent