Re: [PATCH v4 2/5] docs/about/deprecated: Deprecate the qemu-system-i386 binary

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

On Mon, Mar 06, 2023 at 09:46:55AM +0100, Thomas Huth wrote:
> Aside from not supporting KVM on 32-bit hosts, the qemu-system-x86_64
> binary is a proper superset of the qemu-system-i386 binary. With the
> 32-bit host support being deprecated, it is now also possible to
> deprecate the qemu-system-i386 binary.
> 
> With regards to 32-bit KVM support in the x86 Linux kernel,
> the developers confirmed that they do not need a recent
> qemu-system-i386 binary here:
> 
>  https://lore.kernel.org/kvm/Y%2ffkTs5ajFy0hP1U@google.com/
> 
> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> Reviewed-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  docs/about/deprecated.rst | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> index 1ca9dc33d6..c4fcc6b33c 100644
> --- a/docs/about/deprecated.rst
> +++ b/docs/about/deprecated.rst
> @@ -34,6 +34,20 @@ deprecating the build option and no longer defend it in CI. The
>  ``--enable-gcov`` build option remains for analysis test case
>  coverage.
>  
> +``qemu-system-i386`` binary (since 8.0)
> +'''''''''''''''''''''''''''''''''''''''
> +
> +The ``qemu-system-i386`` binary was mainly useful for running with KVM
> +on 32-bit x86 hosts, but most Linux distributions already removed their
> +support for 32-bit x86 kernels, so hardly anybody still needs this. The
> +``qemu-system-x86_64`` binary is a proper superset and can be used to
> +run 32-bit guests by selecting a 32-bit CPU model, including KVM support
> +on x86_64 hosts. Thus users are recommended to reconfigure their systems
> +to use the ``qemu-system-x86_64`` binary instead. If a 32-bit CPU guest
> +environment should be enforced, you can switch off the "long mode" CPU
> +flag, e.g. with ``-cpu max,lm=off``.

I had the idea to check this today and this is not quite sufficient,
because we have code that changes the family/model/stepping for
'max' which is target dependent:

#ifdef TARGET_X86_64
    object_property_set_int(OBJECT(cpu), "family", 15, &error_abort);
    object_property_set_int(OBJECT(cpu), "model", 107, &error_abort);
    object_property_set_int(OBJECT(cpu), "stepping", 1, &error_abort);
#else
    object_property_set_int(OBJECT(cpu), "family", 6, &error_abort);
    object_property_set_int(OBJECT(cpu), "model", 6, &error_abort);
    object_property_set_int(OBJECT(cpu), "stepping", 3, &error_abort);
#endif

The former is a 64-bit AMD model and the latter is a 32-bit model.

Seems LLVM was sensitive to this distinction to some extent:

   https://gitlab.com/qemu-project/qemu/-/issues/191


A further difference is that qemy-system-i686 does not appear to enable
the 'syscall' flag, but I've not figured out where that difference is
coming from in the code.

With regards,
Daniel
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