Re: [PATCH v2 0/3] target/i386: allow using named CPU modeles with user mode emulation

[Paolo Bonzini]

On Mon, Jun 26, 2023 at 11:20 AM Daniel P. Berrangé <berrange@redhat.com> wrote:
> > - for qemu-i386, the issue is that this emulator is restricted to running
> >   programs on a 32-bit processor, even though it is actually emulating
> >   programs that use the 32-bit kernel ABI on any processor.  The behavior
> >   the processor in compatibility (32-bit CS, EFER.LMA=1) mode is mostly
> >   the same as when EFER.LMA=0 and the little code that is needed to handle
> >   64-bit ring-0 is even present in the qemu-i386 binary, just hidden
> >   behind checks for HF_LMA_MASK.  So, LM can also be treated as a feature
> >   that is only relevant in kernel mode; not entirely, so this cannot yet
> >   be extended to bsd-user, but the required changes are minimal.
>
> IMHO in this case we should hard block all named CPUs with
> 'lm' set from qemu-i386. It only makes sense to use named CPU
> models that were actually from the 32-bit era with qemu-i386.
>
> If someone wants Nehalem then they should be using qemu-x86_64.
> If someone wants qemu-i386 then they should be using an older
> named CPU model predating 'lm'.

What you write is true for qemu-system-*, but the problem is that
qemu-user binaries are only able to run one ELF target. qemu-x86_64 is
not able to run i386 binaries, unlike a 64-bit kernel; and that's
really intrinsic in the design of qemu-user because implementing
multiple ABIs (including multiple definitions of structs and syscall
numbers) would require compiling the same files multiple times.

Also, when using qemu-i386 on a (non-x86) 32-bit host you really do
not need the performance penalty of 64-on-32 emulation to run 32-bit
binaries.

Paolo