Re: [RFC PATCH 0/3] Deprecate the qemu-system-i386 binary

[Paolo Bonzini <pbonzini@redhat.com>]

On 4/27/23 10:13, Thomas Huth wrote:
> On 26/04/2023 12.59, Paolo Bonzini wrote:
>> On 4/25/23 15:38, Thomas Huth wrote:
>>> - CPU types have different suffixes between the -x86_64 and -i386
>>>    variant (see TYPE_X86_CPU in cpu-qom.h) ... do we need to care
>>>    about this in the new qemu-system-i386 symlink run mode?
>>>
>>> - The code in target/i386/tcg/sysemu/smm_helper.c looks like it
>>>    maybe needs a runtime switch, too ... or is it ok to leave this
>>>    hard-coded to the x86_64 version?
>>
>> Yes, it would have to switch based on the CPU's LM feature.
> 
> Ok. BTW, what happens if you run qemu-system-x86_64 with -cpu lm=off 
> today? Isn't that a problem already?

Nothing special---I was thinking of migration compatibility from old 
savefiles created with qemu-system-i386, but using LM would break 
compatibility from old savefiles created with qemu-system-x86_64 and 
32-bit CPU models.

A better way to go would be to have a separate property than LM (e.g. 
64bit-smm-format=on|off), and add a "-global" based on -i386 vs. -x86_64.

>>> Anyway, I'd like to get some feedback on this idea here... What
>>> do you think of the idea of getting rid of the qemu-system-i386
>>> binary this way in the future?
>>
>> I wonder if we should take this a step further and rename 
>> qemu-system-x86_64 to qemu-system-x86!  Distros can if they wish 
>> create symlinks to both qemu-system-i386 and qemu-system-x86_64.
>>
>> Then we would name the CPUs "foo-x86" and alias them to foo-x86_64 
>> and, if they don't have LM set, to foo-i386 as well.
> 
> I like the idea! ... we could maybe even go a step further and change 
> the default machine to "q35" in the -x86 binary (or use no default 
> machine at all), and switch back to "pc" as default if running in 
> -x86_64 or -i386 mode...

That's a clever way to move away from the "-M pc" default, indeed.

(BTW, on the user-mode emulation side qemu-i386 and qemu-x86_64 identify 
the ABI, so the binaries cannot be unified.  Still, it would be 
beneficial to make qemu-i386 use the x86_64 emulation code, because 
right now QEMU user-mode emulation---unlike real hardware---cannot run 
32-bit binaries with new CPU models.  So the above unification effort 
would benefit both system and user-mode emulators).

Paolo